Dude, I'm right behind you, go for it, if you don't try it you will probably regret it, don't use your own money, don't use your rents money, don't use friends money, use the banks/investors money.
I got my figures from opening a bike shop just before the recession and also just before the price of bikes to retailers took a 30% (?) hike.
Just putting this to you in case you havn't thought of it. There may be a hole in the market for a reason. I can almost guarantee you are not the only person that has thought of this, is there a halfords? If there isn't ask yourself why. They have teams dedicated to finding places to open up and will have done a very good market research program. Bare in mind cyclists are VERY loyal to their bike shop. Great if you are the shop, shite if you are the competition.