I actually have a business where we were considering offering 30 miles/$1 (it's a very very very high margin business) and hoping folks would buy gift cards and not redeem them.

2.5c/mile sounds like a reasonable price to pay for this. If they buy $500 in gift cards, we would give them 15,000 miles, and it would cost us $375, with a $125 profit against COGS of probably $28 if they bought and $0 if they expired the credit?