I want to build a warrior-type character with some limited spellcasting ability. I have found that the Magewright and Adept NPC classes from the Eberron Campaign Setting fits best.

My question is, is an NPC classes spellcasting cheaper than another spellcaster's

Because CC is already a rules-heavy system, the best answer is no. If the NPC class list is somewhat whimpy, use the 50% column. That is the system's solution for dealing with spellcasting progressions that have limited scope. After all, that's already half-off the recommended price for full-fledged casting!

If you still feel a bit cheated, just add in a few more spells to the spell list.

Also remember that sometimes the best (from an XP standpoint) solution is to figure out if the spells are going to require opposition checks. If so, you really don't want to mess with whimpy spells because they are going to eat up actions. If they are not likely going to require opposition checks (IE most are buffs, heals, friendly-targeted, etc) then often the best solution is to actually take full levels in something lie Cleric but only take 10 of them scattered throughout the class. That way your character gets access to some cool stuff, but the power is limited - which doesn't matter because their all friendly type stuff.

Anyway, long answer to a short question. I'd either use the 50% column (for a 20 level progression) or only buy a limited amount of levels in a full progression class. Either should be able to get you where you want to go.