You need to understand that the animals needs come before yours, and age has nothing to do with it. Do you get an allowance? have a b-day coming up? do odd jobs? Etc. Even Dimmer switches are around 5-10 bucks (you can search how to hook them up on the forums. I dont use them personally so i can't advise you on them).
There are under tank heaters of different varieties. Some are more expensive then others, and you will ALWAYS pay more at a pet store. For a 40B, you'd need a 30-40gal UTH. They come in sizes i think.. Basically you want it to cover 1/3 of the floor of the enclosure. The petsmart ones, zoomed, zilla, cobra-something.. just search online for under tank heaters and you should come up with some more affordable results. There is a thing called flexwatt tape that you can use but flex NEEDS to be used with a thermostat, it can heat up to 140+ unregulated.
If you can, do shopping online or at shows, you really save money. I know when i was 14, if i needed something on the net (and i had no debit/credit hard back then) i talked to them to see if it was ok if they bought it and i gave them cash so we were even.