Ok you inveted them,they could show a little gratitude and help swallow the bill.
I would split it 5 ways,if it is all your stuff you are contributing,tent,cots,stove ect....I would only split it 4 ways(not counting my self),if they don' t like it they can spend the money to buy the gear and time and go on their own,whats the worst that could happen,you are already eating the bill.
Then next year find 4 new guys tell them it will be $300 a pice before you go and you will supplie the gear have fun.