I brew my own as well. Just got back into it, but I'm brewing from malt extract kits which gets you two cases for $30-40. Is all grain the main reason you can get to $18 on your IPA? Any other websites or cost cutting measures you can recommend?
I'd say most of my savings come from brewing All Grain (BIAB). Extract is very expensive.
I get a 52 pound bag of pale 2 row malt for about 40$ (0.79 cents per pound). You can figure a 8-13 lbs grain bill for a 5%+ ABV recipe. Figure 1-3 lbs of specialty grains, depending on style. I get those for about 1.99$ a pound.
For hops, I usually buy by the pound and can get non fancy (i.e., Cascade or Chinook) for around 10-12$ per. Fancier is around 15-20$ per pound.
I culture my own yeast and keep it in the fridge. I figure I can brew about 20 batches (about 4 generations) on 1 sachet. I could stretch it more but I like trying different kinds. Currently using NW-1332.
By the way, this is all in Canadian money, you can probably do it cheaper in the states. The best part of this is that none of the ingredients are taxed as they are considered basic food.