I think the price of the scroll is ok. The basic costs (as for anything) would be cost of materials plus cost of labor. In the case of rituals that's the component cost. But labor? Comprehend language is a 50 gp market price (cost that it took the wizard to scribe it into his book). Assuming (for no good reason) that he casts 10 such rituals a year, and he wants to recover his market price investment in 5 years, that's 1 gp per casting. A detect object spell is twice as expensive per level. So that's 2 gp per casting. Say 3 gp for the caster's time (150% the alchemist wage in 3E) and I would say roughly 5 gp labor cost total for the caster - assuming the adventurer's go to him (travel is extra for time and risk). You could take a multiple of the 5 gp value based on different assumptions (eg. fewer rituals performed per year, rarity of spell casters).
If you charge much more than this then your economics minded PCs might go into the business of casting rituals for others. Also - a wizard who enchants 100 gp of materials to produce a 100 gp magic item isn't making any money for labor. Taking the above figures, you can reduce the cost of materials by 5 gp per day (or fraction) of the ritual - so 95 gp of materials are required to produce a 100 gp item. Only detail oriented players might care about this, but it's not hard to do once you've established a cost for labor.
With regards to "magic item shops" I think such a concept would only make sense in the highest of high magic worlds. I can see most magic items sold on comission, to auction, or to government offices with huge amounts of cash. The King's Armorer might buy +1 swords at a good price (for the king to give out to loyal nobles later). This means taking the "peddlar" approach - and doesn't make a physical building or market stall necessary at all (or useful - I just don't think there are enough people walking around with astral diamonds falling out of their pockets to justify it).
I'd take the reduce treasure parcels approach. Figure out the value of goods that an X level adventurer should have. Figure out the same for X+1. Subtract the second from the first and that's how much treasure you should give out. Of course this will mean that PCs will have to cobble together/sell piles of lesser value stuff in order to get that Sword +6 that they've been wishing for - but your economically minded players shouldn't mind too much. They won't be finding +6 swords in treasure hoards.
