I flip a bike every few weeks. Probably over 200 by now. I sold a GT road bike last week. I paid $180 and sold it for $250 to a happy customer. All I did was adjust the brakes and trim the rear derailleur. Profit is extremely variable as is the time required to produce a quality product. Your time invested is essentially free if you enjoy it and if it puts you in the zen zone, then it's therapeutic and you can decrease the dosing of your psychiatric medication. You could buy a vintage Fuji for $20 and wipe it down with a rag and sell it for $411 (true story) or a Jamis Quest for $350, do absolutely nothing to it, and sell it for $700 a week later (true story). However, you may have a vintage Schwinn that you bought and knew it needed at least 1 tire, a new seat, and a good overhaul..i mean total disassembly...and find out that the stem is stuck and after soaking it for a month it still won't come out and the bottom bracket axle is pitted, so you end up just breaking even selling the parts (true story). Most bikes are comfortably in the middle of these extremes. They will talk to you and let you know what they need. Pre-brifter generation bikes almost always require a complete tear down and inspection and servicing of all moving parts. I've never lost money, and it's nice to take my wife out to dinner and pay cash.