In my case, the grower knows how many colonies he wants or needs to cover his orchard(s) and I try to get as much from him as I can for the service of providing those colonies when he needs them and their removal when he is done w/ them.
I also make sure that they are as strong as they need to be w/out the risk of swarming while in the orchard, which is no good for me and less good for him. I believe it is true that a colony that is about to swarm won't do as good a job as another which isn't on the verge of swarming.
The growers know or should know how many colonies per acre they need or can afford. The cost of pollination service is the lowest per acre expense the grower incurrs. Yet they always seem to get by w/ that.
It seems as though pollination prices very from crop to crop and regionally. It's also a matter of what the market will bear.
Squeak Creek Apiaries