It shouldn't be like a paying gig, it should be a paying gig. Your remuneration will depend on the calibre of the wedding, the price that the main photographer commands, and your skill and experience. Decide in advance what you feel would be a fair hourly rate and don't go below it. Make sure that you have a contract in place with the main photographer that describes your duties, rate of pay, expectation of performance, and image use rights.