Master Franchise Valuation

How best to value a master franchise opportunity?

Existing master franchise
8 stores with 50k in royalties
potential for 8 more stores
Retail and business to business
non-food

Seemingly well run franchise operation with happy owners, low failure rate, and happy master franchisers.

Valuing current revenue stream is pretty straight foward (2-4x income).

What about the potential?

Any ideas, experience will be appreciated.