These are just tiny placeholder changes until a real economy engine goes in. Supply and demand and all that.
Rent: Basically, it checks the value of all your bedrooms. If you have say 100 bedrooms at standard quality, and 100 at say Fine quality, the difference in rent shouldn't be that high. You can easily outmatch any demand of bedrooms with a supply like that, so the prices would be quite low, like only 100 for the standard quality, and 300 for the fine, compared to the 250+ for the standards and 1000+ for the fine at the current rents. Also, having cheap higher quality rooms would allow mega-cheap peasant quarters at like 10-20 rent.
Later in the game it's nigh impossible to make low quality furniture for rooms, because your craftsmen are too skilled. So either you have to skip the quality, have sadder dwarves and slower production by making an unskilled guy a new craftsmen, or suffer having half your pop living in the barracks.
Clothing: It doesn't check the quality, the material, what encrustments/images it might have or nothing. Just the value. So if you have a large supply of expensive dyed silk gloves made from high grade cloth, those would cost a gazillion. Since you are likely to have a large supply of those, the price would be greatly lowered, making clothes affordable for lower grade dwarves. It only checks the fortress owned/store owned items though, not the privately/noble owned ones.
However, don't make it so high demand and low supply raises the prices, that should wait until a full economy is implemented, and other options as being able to select specific levels of crafting and such.
As it is now items can become cheaper if you seem to have alot of them, but it doesn't take into account the extreme quality of the items. If a glove costing 500 is cut by 50% to 250 in price, it's still a wickedly expensive glove, and the dwarf in question would need to buy a pair, and also an entire set of other clothing as well. Meaning he will either have to skip paying rent, or not eat for a year.