Probably about what you paid for it, unless you paid way to much. The nice thing about Mustangs is they appreciate not depreciate.
It's up to you how much you want to spend on it, you can spend more than it will be worth. You also have to decide whether to restore it to original or modernize it.
If it's a nice coupe, it can be worth anywhere from $6k to $10k.depending on the condition and upgrades.
Here is a link from our group that covers desireable conversions
Here is a good support group