A showroom condition, very, very low mileage all OEM yellow survivor Formula, it was either a 77 or 78 - I cannot remember -, sold for around $17K on eBay recently. You would have to compare the condition of your Formula against this benchmark to help gauge its market value. Please bear in mind that the Formula I mentioned was in showroom condition, had lived indoors forever, and looked to be perfect. It was not something that had a lot of miles and use. A showroom Formula with OEM paint and OEM graphics has a more demand than just a nice looking driver quality Formula that has been well used and that sports a ratty interior.
Even though there were only 24K Formulas compared to 93K T/A's produced in 1978, Formulas still do not have the market demand that a T/A has. Maybe that will change someday. I would much rather restore a numbers matching T/A than a Formula for that reason even though I really like Formulas.
If the build sheet has not been removed you will likely find it tucked into the backside of the rear seat, tucked into the bottom of a front seat, behind the plastic over the rear wheel wells, or on top of the gas tank. The locations I have found most common are behind the rear seat and top of the gas tank.
SE clones are popular. A clone from a Formula versus a standard Trans Am will likely not sell for as much since underneath the cosmetic change is a Formula, not a T/A. But if you are cloning for yourself and plan to keep and enjoy for a long time who cares.
These are my opinions only. My best to you.