How much would a kid clubhouse cost?

four 8 year old boys and three 13 year old girls are my neighbors and they want to build a clubhouse. they want to earn enough money thru car washes, donations, jobs (ect.). they want it to be big enough to fit them all in it and extra room. so far they have some decorations. and i was thinkin of getting them a tin roof. any ideas of the cost, how it should look, and anything else?