I've never opened up a gym before, but in my line of work I've dealt w/ city building inspectors quite frequently; so before deciding on buying or leasing a spot, some building expenses to take into consideration are whether or not your local health authority requires the gym to have a commercial ventilation system installed, possible renovations (carpentry, electrical, plumbing etc)/construction permit costs, and the need of a contingency fund in case of cost overrun or emergency maintenance - (usually in construction, the contingency fund is an additional 25% of the initial budget).