I'm looking to upgrade 15 maglights, and I want the best replacement bulb for under $20.
What exactly would constitute "best" for your application? Looking for output at all costs, best runtime with comparable brightness to stock, or what?
As for the ROP suggestion, it seems a little out of place in the LED forum (as does the term "bulb"...), but I guess a WOP (Whimper Of the Pelican) can be made in that price range, $10 for bulb, $10 for high-resistance battery carrier -- and I bet it won't melt the plastic reflector!
But that does raise the question of batteries -- are you planning on rechargeables, alkalines, or what? You'd spend at least $20 on 3 NiMH D cells, a little less for NiCd. (I guess you might get less in either case, since you'll be buying in large quantities.) If a ROP is actually interesting, and you're planning on rechargeables anyway, you could get 6x 4/5SC for a little less than 3x D, and spend $10 on bulbs, and $10 for a KD reflector. (Plus I guess a piece of pipe, so just over $20 -- take that out of the battery saviings.) Most other battery options wind up costing more, though.