Metamagic rods.
How much gold do you have? What PC level are you?
From the example you made (ring of protection +1) I believe you are low level, if the amount of money you have in your hands is around 2 thousands gold then your best bet is buying a pearl of power and some scrolls (you never know when they can come in handy), save the rest for future purchases, when you have enough metemagic rods are really powerful (possibly too powerful).
Merciful Metamagic Rod and a Pearl/Runestone of Power or handful of useful scrolls. The former is surprisingly useful for gathering information (assuming your party/fellow players actually need to know more about what they are doing) but can be skipped if you rarely/never have to take prisoners. The latter 2 are often more useful than anything else. I advise always having a scroll or oil of silence and one of enlarge person if possible. Maybe a wand of CLW/Infernal Healing if the party doesn't use a group fund for healing. None of these will directly help your damage output, but all of them are useful.
Other good semi-cheap intend not related to your spells, include handy haversack (or carrying capacity be gone), boots of the cat (for when you need to take the quick way past an avalanche), feather token tree (instant cover, escape swallow whole, drop on enemies below you, maybe break a buildung or two, etc), alchemical frost/acid (swarms are annoying), flour sacks, Traveler's Any-Tool (the batman utility belt), and Sleeves of Many Garments (for when you need a new dress RIGHT NOW!) Can't afford them all, but you probably don't need them all either.
Look for items that raise caster levels.
http://www.d20pfsrd.com/magic-items/wondrous-items/wondrous-items/r-z/robe- blazing
Make your fireballs 1CL higher. etc.
You could ask your GM to take a look at the esoteric spell component subsystem from Pathfinder Unchained.
I think that the OPs question is being missed. It's something I've always desired for my mage characters too.
Not metamagics, not utility items, not even limited caster levels...
...an item that actually gives you a bonus to the DC or even the damage (bonus damage per die would be cool). Everyone else can get items to do more damage with item spends! I do get it...but your damage goes up with your level! Yes, scaling damage is good, but I still want more! 10d6 fireball does sound good, but at 10th level, enemies are resisting energy attack,making their save and even evading! I want more!
I want a staff that I can focus my own magic through to make it stronger! I want a ring that I can channel my energy through and change the element as I require...I want more!
I think that the OPs question is being missed. It's something I've always desired for my mage characters too.
Not metamagics, not utility items, not even limited caster levels...
...an item that actually gives you a bonus to the DC or even the damage (bonus damage per die would be cool). Everyone else can get items to do more damage with item spends! I do get it...but your damage goes up with your level! Yes, scaling damage is good, but I still want more! 10d6 fireball does sound good, but at 10th level, enemies are resisting energy attack,making their save and even evading! I want more!
Like I said, look at the esoteric spell component optional rules in Pathfinder Unchained.
I think that the OPs question is being missed. It's something I've always desired for my mage characters too.
Not metamagics, not utility items, not even limited caster levels...
...an item that actually gives you a bonus to the DC or even the damage (bonus damage per die would be cool). Everyone else can get items to do more damage with item spends! I do get it...but your damage goes up with your level! Yes, scaling damage is good, but I still want more! 10d6 fireball does sound good, but at 10th level, enemies are resisting energy attack,making their save and even evading! I want more!
Well Feats are there for a reason.
You want to be a caster and benefit without spending feats, traits or anything else other than gold?
Even fighters have to spend feats to up damage and to hit.
Basically power attack is a must for them to get more damage much like Empower or Intensify gets you more damage.
Weapon Focus gives the fighter better to Hit, Just like Spell Focus gives you increased DC so your spells effect more targets.
I truly believe the OPs question was answered:
I'm wondering if there's a way to empower spell's damage or duration using objects. I mean... I'm playing a suddenly I found myself with a considerable ammount of gold.
People have said Rods, Alchemical Items, and a few wondrous Items.
Other option would be to dip a class like Cross-blooded Sorcerer Orc/Dragon for +2 damage per dice rolled. Turning your 10d6 fireball into 10d6+20.
So yes the OP question was very properly answered.
Gosh...take things seriously much?
Nope, still not the point though...
Weapon focus = spell focus...yep, but not talking feats.
Not limited optional magic systems...that I would end up having to persuade a DM to change his game's magic system to accommodate me!
Not rods that give me a limited metamagic feat I probably didn't want because I didn't choose it...and see above, not talking feats.
Not going cheddar by dipping into a completely nonsensical character class, to get my extra power - cross-blooded sorcerer orc/dragon, I mean, seriously!
No I am after an item that makes me more effective across the board, all the time.
Like picking up a +3 longsword. +3 to every hit roll and damage roll I do...
I would love to play a blaster wizard that could actually be effective without focusing on magic missile or disintegrate...
Well Increasing your Casting stat like a headband of INT +4 will increase your Spell DC by 2. Means it is Less likely to get resisted and the enemy will fail the save more often.
The difference in Damage between a Successful save verse a Failed one is HALF damage. (0 damage if Evasion is involved). That is Every time the spell is cast much like the +3 Longsword being useful on every swing.
Accuracy (or Spell DC) goes hand in Hand with damage potential.
A fighter who misses does 0 damage. A Rogue who passes his reflex save against your fireball takes 0 damage. So the goal is to make it so your spells stick as much as possible.
I think your just wanting something that is already there but un-happy with the way they are presented.
Also the Pathfinder System rewards specialization over generalist style characters. This means specialized characters actually perform better in actual play. You may not like it but that is how the game is designed.
Now if you want special items that do not exist inside the game then talk to your DM/GM on the issue and work up something with them. But I believe the OPer wanted options that actually exist.