If your Festae is in great condition, I would say $50.
It is a large, agressive fish to mix, so there really aren't that many customers out there (which is why many shops don't take big aggressive cichlids in, or won't give you anything in exhcange). Unless someone here on this site wants to buy him off your hands, that is.
If you do want to keep him, try getting some small caves that he can't get into or overturn. A big, heavy piece of driftwood or rock that allows your smaller fish some hiding room away from him might help. As well, don't introduce 1 fish. I would throw 4-6 fish in there all at once.
It really is tough to judge what you can or can not put in there with him. Festaes, like most large 'central american' (I know he ain't a central, but same 'similar disposition to them, I would say) are true individuals. In my tank, they are the weakest fish. I have 4 of them between 5" and 8" now, and all 4 of the small red devils (labiatusXcitrnellus) dominate them, as does my 5" tuba, and my 5" Dat if you can believe it.
If you really like him, keep him and try it out, even if you try it with cheaper convicts to see what happens. They sure don't come around all that often, so IMO, it is a huge waste IF you like him.