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

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.