Prior to recently purchasing the building that my new salon is in, I bought the lease for the commercial element of the property. (I didn't want a building that I couldn't use for my business for 9 years!)
I spoke to a friend who is a local estate agent prior to making an offer for the lease. I was advised that the cost of buying the lease ('key money') would be in the region of 6 to 12 months multiplied by the monthly rent. 6 months if trading is not too great (recession) and 12 months if business is good (boom times). As we are in a bit of a recession I managed to get the lease for 6 times the monthly rental figure.
Do however check in your locality, as things could be different in your region.