When I worked at a bike store, the margins were something like this: 50% on a lower end bike and 100% on accessories.

You might read that to say there is lots of room on the bikes, but in my experience there is not. There are a lot of costs associated with the bikes (having to have them there, lots of sizes, etc etc). The most I think I ever went down to for a customer was selling it at a 40% margin. However, I didn't work at the end of the year, so I don't know what they do then. But, assume that the regular price is at about 50% margin, and see how much the discount is already.

I'd totally give a customer deals on accessories if they bought a bike. Even at 40% off we're still making money.

Also, I will say that using a Visa costs the store something like 2% automatically. American Express might be more. Debit was like $0.30 a transaction, regardless of what the amount was. So if you bring cash or debit you might be able to get a few bucks off, but don't try and be cheap for $50. You'll make no friends at the store and having friends there saves you a lot more than that in the long run.