if he's willing to pay up to $360, the rest of you will be paying something like $300... $60/month for a bathroom seems fair to me. A few of my buddies were recently in the same exact situation. The kid who took the bedroom with bath is paying $50-60 more per month for it, which is fair. if the three of you don't think it's fair, then toss him out, one of you take the master bedroom, and find another person to take over one of the other bedrooms...
depending on what city you're in, use of the garage could go from $25/mo to $60/mo...