When I buy off craigslist, I pay 1$ per gallon maximum, you can find them cheaper though, so I wouldn't pay more than 30$ for just a 30 gal.tank. After I build my rack ( in progress, I will sell for same) You probably still need to get a screened lid and clamps to lock it down.
If you are keeping a cornsnake you don't want a heat lamp, you want a UTH ( Under the Tank Heater). good luck