I'd estimate somewhere between $16-18K max. It'd be difficult to ask for $20K for an EVO3 re-shell when you can spend another $4-5K and get a genuine JDM EVO6 or 7 if you look hard enough.
Depends on what the buyer is after but I reckon the hard spot you're in is that somebody wanting an EVO3 and prepared to spend big $$$ on one is probably wanting a genuine yellow plate EVO3.