(side note: it's "Tarot", fyi).
This item is unique enough that there really is not a comparable item (to my knowledge) to compare it against. Maybe a Necklace of Fireballs might be close (not a "bag", but same general idea). Also a Bag of Tricks has similar use, but considerably less power.
I have a few questions:
1) Are the Runestones reusable? Can he pick them up after the fight and put them back in the bag?
2) Are the Runestones inexhaustible? In other words, is it like a Bag of Tricks that never runs out of its (random) contents?
If the answer to either of the above questions is YES, then this item is a permanent and very useful, fairly powerful item. It's gonna be costly. But if the answer to both is NO, then basically the character is making one-shot bombs (mechanically equivalent to a potion or scroll).
So, assuming the Runestones are not reusable, I would create each Runestone like a potion (but would require Craft Wondrous Item): Spell Level x Caster Level x 50gp. For example, a level 5 Wall spell would be: 5 x 9 x 50 = 2,250 gp base cost (half that amount to make it). I would let the player put as many in a bag as he wants but he has to keep track of how many there are (amount of each runestone and total runestones). Then he just rolls a die to randomly see which one he gets. For example, if he has 4 Wall of Fire, 2 Wall of Force, 7 Wall of Stone, and 1 Wall of Ice Runestones, that's 14 total Runestones, so I would just have him roll a d20 and ignore results of 15-20 (re-roll) - if he rolled a 7, then he drew a Wall of Stone. I would allow him to draw one stone and throw it as a standard action, or draw one stone and look at it as a move action (so he could decide to put it back and draw again). Of course, if only the Runestones are magical, he could just as easily keep each type in a separate pouch on a bandolier and always draw exactly what he wants, which eliminates the randomness.
On the other hand, if the Runestones are permanent or if the bag never runs out of Runestones, then I would treat this item exactly like a magical Staff. I would use the same rules for creating a Staff, except he's making a magical Bag - but mechanically it should be exactly the same thing - 10 max charges, multiple spell abilities, and a recharge mechanic that lets him replenish Runestones by spending spell slots. In fact, it would not matter to me whether he called it a Bag of Runestones or a Staff of Walls, the game mechanics would be identical - the only thing different is the fluff about what it looks like and how he uses it. This gives him a huge advantage in that the Bag of Runestones would use his caster level rather than the (very low) preset caster level that wondrous items normally use.
If you use that last option, it takes the "random" element out - staves don't require the user to to randomly roll for which spell he uses. If you want to add the randomness back in, you might simply reduce the casting cost of the Bag of Runestones by, say, 20%. I wouldn't go lower than that since he knows, at least, that he will always get a wall.
I hope that helps.
For a different take, you could also build it as a X/day use-activated item for the highest level spell that come out of the bag. The chance of getting something weaker than that would be worth a 0%-30% discount depending on how many spells there are, how situational they are and what the lowest level spell is (in other words, the GM would have to guess). If you counted each wall as heightened to the level of the highest one, I wouldn't give more than a 15% or 20% discount. A bag like this would generate or enchant up to X stones per day and you'd have no way to control which ones you got out. (I'd reduce the discount a bit if you rolled at the beginning of the day rather than when you drew them.)
Another version would be to price it as 1/day for each stone. Then the bag is actually just a bag and it's the stones that are magical and each can be used once per day. (Probably with the same discount if something forced you to chose one randomly.)
Yeah. All the formula for each of these options are in the magic item creation table. The discounts are ad-hoc though.
He could just use his Scribe Scroll feat to create runes of Wall of Fire and put them in a bag. Spell runes are basically scrolls. In fact, several published modules have runes as treasure that function like scrolls. It might be much cheaper for him to do that.
Going from the magic item values table, an item that casts wall of fire 3 times per day will have a value of 33600gp.
use activated: 2000 * spell level * caster level
charges per day: / (5 / charges per day )
2000 * 4 * 7 / (5 / 3) = 33600
So the bag as a magic item would cost him 16800gp whereas a mundane bag full of scroll-runes would cost 350gp per rune. It also means that he would have to use the magical bag more than 48 times (16 days if used three times a day) before it would be more cost effective than the Scribe Scroll approach.