Adding paid public WiFi service to a restaurant

We plan to add paid public WiFi access to our existing wireless router,
which so far has been used only for internal purposes.

After searching the Web, NetNearU (affiliated with Boingo) looked like a
good choice. NetNearU advertises no upfront cost to get started, but
once you've paid a processing fee as part of the signup process they
drop the news that you have to pay for an expensive wireless router at
ten times the normal retail price. No option to re-flash our existing
router.

Is there a solution that's less expensive? We'd rather not spend
hundreds of dollars upfront since we don't know whether we can attract
enough paid traffic to cover the cost.