If you mean the current reissue, it goes for $ 750 new, I would expect about $ 400 used if it’s in good condition. If this is a vintage Mustang from the 60s or 70s, it can run anywhere from $ 450 to $ 1350 depending on condition.

Not enough detail, but a Mustang bass would probably get around $ 500 – $ 600 depending on condition (unless it’s actually a Squier, not a real Fender in which case you’d get about $ 150 tops for it). If it’s a vintage one you might get over $ 1000 for it. You’d have to specify the year, model number, and condition to get a more accurate quote. On the amp, you didn’t give any details, so it’s impossible to say.