It's not really that they're cheapest... I paid $20 for the ones from a local place... It's more like I believe they'll actually do a good job, and they can help with the layout and design stuff as well. Ronnie Gamble is a freelance graphic designer with a solid portfolio, who happens to own one of the coolest 40K armies I've ever seen (The Mariner's Blight. Look them up sometime). So, I'll gladly pay someone who understands the hobby make my decals, since he's much more likely to care and pay attention. Yeah, it's pricey, but a sheet of decals goes a long way and for custom work, it's pretty well inline with what I've seen.
The $20 might seem cheap, but I did all the layout and gave him printer-ready files and even the paper to use. He literally just pushed the button, using his own paper for some reason (claiming his printer can't print edge-to-edge, which it can and I gave him gutters anyway), and then to my chagrin he went ahead and sprayed the sheet with gloss coat even though I told him not to. The local guy also lost my files twice, and only started the work after I called him twice. So, you can see, bad experiences are not worth saving money. At least in my opinion.