If you shop the net you can find Diamond hole bits cheap, and they work. I have drilled several tanks and we just did a demo at your last club meeting. If the LFS charges 30-50 a hole I would look into buying a bit. Bottoms on most tanks are tempered so you can't drill them. check with the company that made your tank too see if its all tempered before drilling.
If you do drill plan on where you want the holes and take your time use plenty of water to keep the bit cool and for lubrication. keep the drill straight is very important do not bend or twist it, you can crack the tank if its gets teisted in the hole.
My buddy made a drill guide out of a piece of 2x4, he cut a hole in the wood slightly larger then the diamond bit and then put another on the under side and clamped the 2 together. he added lots of water and took his time. It can be done just go slow.