Go for an induction and ask them how many members they have and their costs, calculate their members as a percentage of the local community and then look at the National Audit Office or just Google 'how many people have gym memberships', here's a link:-
http://www.mintel.com/press-centre/press-releases/458/boom-time-for-budget-exerciseI've spent the last 6 months doing exactly the same thing, but came unstuck when my potential business partner wasn't prepared to put any operating capital in and he wanted me to take all of the risk, which I wasn't prepared to do. Good luck with this and don't forget to be realistic with your growth projections and he time of year you get started will have a big impact too. Sorry to preach but I have quite a lot of background to this stuff and I'd be happy for you to email me directly if you wanted.
Top tip :-
Classes, classes and classes = bigger profit margins
Rent floor space to PT's and as part of the deal they give you hours (eg 16hrs) on the gym floor = less staffing overheads
With regard costs, you'd need at least £30k start up capital for a bank to take you seriously and a 5000 sq ft venue to make it unique to the others and seriously as much time needs thinking about ancillary areas as the gym floor, changing rooms, coffee area, interview room, staff rest area (planning may require one), disabled access etc.
Exiting times, but a lot of planning!
Hope this helps!
Neil aka Ginge