There’s no good answer for this… I’ve done freebie stuff for friends and nonprofits and activist groups, all of whom have either said or implied that "it will give you great exposure". In the beginning I believed that might be the case, but I can’t say any real work has ever come from these things — not saying it can’t.
Sometimes they can go into your portfolio though, and the benefit is that you might get to experiment and do the kind or style of work that you want to do more than you might if they were a paying client, which can lead to those types of clients seeing your work in your portfolio and wanting to hire you based on it.
Depends on how busy you are at the moment too. If you’re in a dry spell or are just starting out and need to fill your portfolio, it might be worth it, but try to get something in return — if not money then their product or service or what have you (if you have no use for it yourself, maybe you know someone who does and can retrade it or whatever) or a promise to pay later if things go well, and of course if they’re offering to promote you in any way, get that too. Get all you reasonably can out of them if payment isn’t forthcoming.
Otherwise, they get something for nothing, and that drives what I’ve come to call the "Craigslist mindset" that good design is, if not free, then close to it, which brings us all down.
In your situation, it sounds like you’re getting some payment at least, so if the money to "exposure" ratio works for you there in the short term and you decide it’d be a good portfolio piece as well, maybe you can either set up a payment plan to make it more manageable for them or set a milestone for their site/business somehow and have them agree to pay the rest at some future point, prefaced strongly by "I wouldn’t normally do this, but…".
(If its at all related to multi-level marketing, just run. Fast.)