In most industries, the standard markup at retail is 50% or higher above your cost. Factor in buying the bike, and all the money you put towards it. Most retail stores actually mark up things 100% minimu and this goes for everything from clothing to shampoo to booze. Don't short yourself just cause you're an alternative. Of course you'll have some cheapo stuff, but if you've got real nice older bikes, get an idea of what they're worth via bike chatrooms, forums, craigslist, ebay and the like and then do your thing. You should research what a bike's approx worth is before spending money on the upgrades so you'll know if upgrading is worth it or not. Many bikes are probably worth their cash simply put together, let the customer pay for the upgrades.
Just my 3 cents