read MFK. i spend less money running 3 large tanks than when i started out with a 38g simply by reading all the info on this site, and no other. it used to cost me atleast 15-20$ a week with my 38g, and now with a 150g, 100g, and 75g im proboably spending 5$ a week. the money i spend on fish now is insane though!
Keep your fish tank away from cold draft from window or door creaks. This will reduce the amount of energy needed to keep your tank water at a constant temperature. Use hot water to mix with cold water in your waterchange bin when doing a waterchange. It cost less to use hot tap water than to use a 300 watts electric heater to heat your water up 10-15F degrees.
Join a local fish club and you will save loads of cash. Most clubs have a private auction where you can get fish 1/3 the price of retail. Raffles are great too. In my last raffle, I won 2-3 months of staple food all for the cost of five $1 tickets.
DIY, DIY, DIY. Fix broken tanks that most would simply give away. Build your own wet/dry sump. Build your own stand. These items are the largest (pricewise) that you will aquire in the hobby. Doing it yourself will save you hundreds of dollars. The information for these is all here and all over the interweb.
I know personally, I aquired a 190 (5'x2'x30") in a trade for a Craftsman tool chest I bought off my neighbor for $40. The deal included a 50 gallon wet/dry sump. Purchasing new, this would've easily been close to $1,000. All for $40.
Keep hunting Craigslist til you find what you wnt for what you would like to pay.
Go Fishing! Use your catch for food. Cut the fillets into little portions and throw them in for your fish. They'll love it! Plus you have an excuse to go fishing too! Its a WIN-WIN situation!