It's funny. I remember a time when I could buy 1,5 liters of coke for half of what a 0,5 liter bottle costs these days. Yet computer games cost more then than they do now. So that in some places, the prices have gone down even without adjusting for inflation... While, as mentioned, budgets have gone down. I'll admit that the inflated prices of the early nineties might have had more to do with small local retailers scalping in a closed market, but still...
p.s. I don't think it's possible to treat all DLC as whole. Sometimes it's a blatant rip-off, sometimes it's cut content that wouldn't have been implemented within the regular game budget, and sometimes it's a team allowed to expand the game further later on. I'm not seeing much of the first group with RPGs, mostly with the "map packs" for certain popular shooters.