At this time I am not showing -- either self show or trainer.
However, when I did show several yrs ago and usually 2-4 at a time, I can tell you that the ONE THING which was worth every penny was my trainer!!
I actually ruffled his feathers a little one weekend when I inquired of other trainers what their fees were per horse -- naturally, those trainers went to him and said "hey, she's shopping!"
Couldn't have been further from the facts......I was shopping to see if I was paying enough
So, I then presented him with a contract to pay a BONUS to them based on certain criteria. Let me tell you, he did get bonus money!!!
And I was happy to pay it. Great horses were provided but, the work it takes to make them all that they can be is valuable. My animals were always treated well and presented beautifully. I was new to mini shows and at the time, so was my trainer just starting up a string -- most know him, Lee Crutchfield... he is family to me!
Hubby & I went to most shows and helped where we could (often just leading, helping bathe, etc) and I made it a point to keep my trainers FED. There just was no time between classes with several client horses to get what you should -- let's face it, burgers are not good every day! -- so I always packed my motor home with healthy snacks and lots of homemade meals. I saw to it that good food was provided during the day. It was one of the things that made me feel like I was "doing my share".
So -- I'm not sure what kind of "co-op" you are thinking but, a good trainer deserves paid.
Now, if you are thinking along the lines of a "co-op" that might be benefit to share transport expenses by sharing a trailer, or group training, human meals, etc., you may have some takers. Look to a local club and see what is going on with clinics or other farms/owners, close by that may want to share experiences, etc.