Ribbing can be as expensive or frugal an activity as you like, but thats not what you want to hear is it.
Eight people comfortably on a RIB suggests quite a big boat, certainly about 6m I would say.
For insurance, servicing, minor repairs allow at least a grand a year. Then you have the costs per use like fuel, slipping/berthing etc. And then you have the various additions you will "need" to buy.