A lot of things factor into the answer for this question.
The most valuable Formulas are those with T-Tops and the Getrag 5spd. This was a pretty desirable model, and will fetch more than it is really worth from the enthusiast market. Mileage also plays a huge factor. I would say a T-top 5 speed formula around 100k miles would fetch anywhere from $3000 - $6000 depending on condition of paint, interior, mechanical condition yada yada.
The value drops dramatically without the T-tops expect prices more in the $2000 - $4500 range for a good condition relatively low mileage car.
Automatic takes that down a little further. I have seen Automatic solid roof formulas go as low as $1500 in decent shape but will often fetch around $4000
There are of course factors that could drastically increase or decrease any of these prices. Like always garaged super clean super low mileage T-top formulas have fetched upwards of $10,000 - $15,000 but those are rare "survivor" show quality cars.