I got a bike for FREE (Fun Bike) and probably put oh, $100+ into it. Worth every penny. It needed new cables, new innertubes, tires and brake pads, my now-DH got me shifters for it. I also added a rear rack and fenders, got new peg type platform pedals for it and a red bell and a bike computer for it.

It's my favourite bike to ride currently.

If it is a good bicycle, the amount you paid for it shouldn't influence how much you spend fixing it up. If it has severe problems...might be different. Also, how is the bottom bracket...?