I find that I sort of triage the bikes I buy. Some of the better ones (say Schwinn World Sport or above) will get the full rebuild treatment of cleaning, new parts and adjustments. Cheapo bikes, like department store road/mountain or low end 3 speeds, will get good tires, brake pads a quick squirt of fresh grease into the bearings and an adjustment. If the cables are in poor shape I will replace them, but I leave the outer housing if it is in reasonable shape.
A fully 'dressed' bike will sell for $150-250 and those are worth putting some time in. The cheapo bikes sell for under $100 (sometimes well under) but they are normally going to college students looking for a crappy bike for campus that is going to get left outside anyway. I figure with the minimal work I do on the cheapies, they are still in far better shape than a new $60 bike from Walmart.