As with any costume, decide if 501st accuracy is desired. I'll assume so. Then figure out how much you plan to do the work yourself. The more you DIY, the more money you save, but the more time it takes.
If you create everything yourself, from cardboard, fiberglass, cloth etc. Fett isn't all that expensive. It's just massively time consuming. If you plan on not building a single thing, and would prefer to just buy all the parts from known craftsman that put together awesome parts expertly, then Fett is a massively expensive build.
Most go somewhere in the middle. Buy a helmet, but paint it themselves. Buy the flight suit, but weather it themselves. Maybe pick up a used part or two from the cargo hold or RPF.
After that, it's time to decide on which version of Fett you are doing. Then read the WIP's to find out either/or, how to make or who to buy the parts from, for your costume.
This site is amazing in that there are DIY's for almost every part of the costume, but there are also multitalented vendors on the site that sell a nearly perfect representation of each part.
In most cases, a Fett build is a marathon, not a sprint. Take your time and enjoy the journey of it all. You may only put together a Fett costume once in your life, so you may as well enjoy the process.