I actually think the set costs have been very good as of late, not just in the Castle line, but the Indy sets, the Agents sets; the prices all looked great to me.
It seems cheap sets are including a heaping serving of minifigs. Take any of these three for example:
$6.00 - 41 Parts, 2 Crownies, 1 Undead Skellie, and a Skeletal Horse. This set was (And still is) an incredible army builder. At six bucks a pop, how does one resist?
Dwarves Mine Defender (As mentioned above):
$10.00 - 73 Parts, 1 Dwarf, 2 Orcs. I bought seven last week, mostly to part out and sell, but I can tell this is going to be a big seller (To me, at least) this year. An awesome way to boost your Orc Army & your troop of Dwarves.
Jungle Dual (Indiana Jones):
$10.00 - 78 Parts, 3 Minifigs (Indiana, Mutt, & Irina).
This set includes Other Great parts (Like the new Ant piece) as well. A great way to stock up on some nice new elements.
So, are the prices fair?
To me, yes. And if they had to go up in cost, ah well, I'd still buy 'em (I pay extra when at TRU anyways).