Performance sometimes has $5 cork bar tape deals.
I try to make not less than $100 on a bike, preferably more. It helps if you can get parts near distributor cost. I have an arrangement with a couple shops where they get me parts at very little mark up. Because of this I can put a new wheelset on a bike and mark it up accordingly, without breaking the bank and nullifying profits, or overpricing the bike.
Since I'm diligent about fully overhauling anything used, putting a new wheelset on a bike saves me some time because i don't have to open the hubs up, and it also looks great with new tires. Makes a great impression. Not to mention that at least 50% of the araya aluminums on most of these bikes are wavy, need retensioning, or are beyond repair. I also offer to build custom wheels as a way to make the bike even nicer, and make some more on it. I'm all about taking old frames and components and making them as nice as they can be within a budget.
As some have mentioned, it definitely pays to buy bulk and get deals in the process. Buy lots of bar tape, cables, tires, etc. I ALWAYS put new tires, cables and housing, and bar tape on a bike. I ALWAYS overhaul the headset, and either overhaul the bb, or plug in a cartridge. If the wheels are used, they get overhauled and tensioned. If new, just adjusted and tensioned.