My rule of thumb on "flips" is to make sure all the contact points are "nice" Clean or new grips and saddles. If the cables/housing aren't too bad, I'll give them a quick sand and reuse with some lube on them and inside the housings. BBs and hubs get adjusted and if they're really dry sounding, I'll open them enough to squirt grease in before adjusting. True the wheels, file the oxidation off the brake pads. Take it out and test ride it hard.
Basically the exact same thing I used to do in a shop that charged $80, although they didn't approve of opening a hub to squirt grease in. That was an upcharge.
Most of my flips are purchased for under $50 and sold for $150-250 in a relatively hot market area.
As a popular local latenight gun store commerical when I was a kid put it, "I'm not trying to make money selling *bikes*, I just love to sell *bikes*"