not sure what you mean by "12os are 450" 120 gallon tanks cost $450?
Give us a little more information. What reading you've done, what you're planning on making it out of, will this just be a box or will you build in overflows, etc.
We need a few more detail before we can offer much detailed advice. Read through the forums here, there are a few threads on folks building their own tanks. read through them and get a few ideas, it will also lead you to more specific questions that you can post here and we can help you with.