I was looking at a web site that rents Power Trac equipment. They charge about $20 per day for a stump grinder for example. This could be a way to be fair about the situation. I borrow implements for two reasons. 1. I want to test it before I buy. 2. I won't need it often enough to justify purchase. If the Co-OP had fixed fees like these it would help manage the wear factor. The trencher is rated for one mile. At $1200 (I think that was the price) this is about 23 cents per foot. This could be another way to price it. For my purposes being able to borrow (at a nominal fee) saves the initial purchase price and storage.
I do all the work for my neighbors for fee, so please don't think that I am driven only by money. These are just suggestions and comments that I believe will give you your best chance of keeping a co-op going for a long time.
Again good luck and best wishes for a successful co-op.
Bob Rip