GM DarkLightHitomi~
I'm gonna try to be brief so that this debate doesn't get into huge paragraphs...
1 & 2 . The price of magical items and other costs is indicative of whatever the DM determines. Take, for example, a scroll for 10 gp. A DM can look at that and say: "In my campaign world, rituals are hard to come by and magic is rare. Therefor it means peasants and commoners make only a few coppers a day, up to a silver or 3 a week. This means that merchants and artisans might make more and delve into mostly silver and a few gold pieces a week. Wealthy merchants, nobles, and PCs (ie. Adventurers) are the ones who can dabble in more rare items and magics. What rituals are available, who they can be purchased from and where are ENTIRELY based on the DMs purview. The game makes most of the prices set mostly on the balanced game mechanics. It's the DMs job to figure out how and why these are placed into his campaign, if at all. Some people like that, others don't. It's not objectively a "good" or "bad" thing.
Also, Nobles use Rituals for all sorts of things. It's a roleplaying game, I'm sure a noble on the road is going to want to use Create Campsite or Dark Bridge or Tenser's Floating Disk (to carry their more luxurious luggage). OFten times a wealthy or noble's need to such rituals are no greater than that of a PC's.
3. I'm not entirely sure if a level 8 encounter is even feasible for Level 3's to take on unless the whole party rolled pretty high. A level 3 Fighter (Weaponmaster) with a starting Strength of 20 and a +3 proficiency weapon has at most +11 if we include magical item +1 and some sort of attack-boosting feat (Master at Arms, for example). Now compare this to level 8 standard soldiers and we're looking at requiring a roll of 12 minimum to hit their AC. Now often times that's not a problem and a level 8 encounter of minions might not be difficult. Further tactics DO play a part in combats and when DMs don't use the guidelines FOR such encounters, they often are easier for PCs to tackle. Conversely, I had a group of PCs nearly all die from Kobolds at first level because I stuck to their suggestion on how to run the encounter. Some lucky swings on the Kobolds part and it was nearly a TPK. But, for the most part, Encounter Building works on-par for the majority of the time I've played the game.
4. Tools are already there. You want to impress a crowed, roll a Charisma check (Mod + 1/2 level) and beat X DC. A crowed in a seedy bar is going to be tougher than a laid-back tavern in a good part of town. If your backstory (ya'know, those things people create when building their character) has that you play some sort of instrument or are a decent singer, I'll give you a minimal bonus for the effort. But, in all honestly, to me it's far more about the story your trying to tell than showing off how well someone can play with the system mechanics.
5. When I run d20 modern, I don't involve encounters or problems that can be overcome by trivial things like technology or cars. I ran a Resident Evil d20 Modern game and cars were everywhere. Gas, however, wasn't. You want to use a car for a fast get-away from some 'Lickers', go ahead but don't rely too much on it because rust, being out in the elements, gas, and other mechanical problems can occur. Same thing with magic, it's usable but I'd venture to say that the game creates more suspense and fun when it's not a 100% automatic thing.
6 & 7 (or 8 if you will). Inconsistency is something that is subjective. For you, 10gp a Ritual use is too small for wealthy people BUT too expensive for common people. You find this inconsistent. I, however, don't. I think 10gp is about right for a quasi-medieval setting where commoners can earn a few silvers a week, merchants and nobles can earn gold and it's these people and PCs that continue to keep the magical item economy in business going. I also don't subscribe to the idea or notion that 90% of the populace are commoners or non-wealthy individuals. Aside from the economy of the game, other effects that function from combat are pretty reasonable too. The assumptions on wealth greatly depend on each edition because magical prices change from each edition. For example, a +1 Magical Longsword in 3rd Edition / Pathfinder is 2,315 gp. A 4E +1 common Magical Longsword is 360 gp. You can't make the assumption that wealth of people from 3E should be the same from 4E when the prices are so extreme.
9. The cost for things shouldn't change, but the way in which these economics impact your specific campaign should. 50 gp for plate armor is a easy indicator to vary how much common / merchants / nobles - PCs make.