Walmart offers the Nilight 12V LED Light Bar Wiring Harness Kit for $8.41. Opt for in-store pickup to avoid the $5.99 shipping fee. (Amazon charges the same with free shipping for Prime members.) That's the lowest price we could find by a buck. It is compatible with all automobiles, boats, and devices equipped with a 12-volt power source.

Update: It's no longer available with in-store pickup at Walmart; however, Amazon still offers it for the same price with free shipping with Prime members.