We belong to a co-op in our small town and enjoy it. It is a good deal for us both financially and nutritionally, it's good go the town to have groceries available downtown and not just at the big grocery store and Walmart, and it's good for the people who run the co-op and provide things you can't get elsewhere in town.
Regarding the co-op you are thinking of joining, do you know the details? What are you getting for that $100? Our buy-in was $10 a year membership fee for 5 years, which then transfers into lifetime membership. In return I get a discount, I think it's around 5%, off the already good prices, and we are friends with the people who work there instead of just patrons. We also order a lot of bulk organic foods (wheat, peanuts, whole wheat pasta, etc) at good prices.
$100 initial buy-in seems really steep. What if you decide you don't like shopping there for some reason? How much of a discount does that provide, is it worth it financially vs just purchasing as an outside buyer? Might want to wait until you try it a few times before buying in, but then maybe your initial support of $100 will make sure they do a good job and succeed. Just makes sure you do some research, and I suggest going to a co-op meeting just to see who you are working with and how things are being done.