Depends how the LEDs are:
If you mass buy, I wouldnt expect to pay more than a few bucks each. You figure, connector = 50 cents or less, wire = 20 cents, LEDs = 50 cents each. Although it may cost you less for the LEDs and more for the connectors or something, it balances. Since there's little work involved, Id probably ballpark it at
3 LED harness = $4.00
6 LED harness = $5.50
7 LED harness = $6.00
Plus shipping of course. When you mass buy though, the costs go much lower, so 4 bucks for a 3 LED harness would give you probably 50% profit. If there's sufficient interest though. Im sure if people only need one, they could make their own. Now if there's a special header connection needed, then that could be different. You and buddabing could work together so he needs a harness and you supply it, although that would kinda suck.