rather than multiplier I would go for a Discount Cash Flow.
Cash Flow to Equity over last year. This is Investor's bank account increment over 12 months
Discount Rate: this is the return on investment which reflects the risk of the CF
Period in years where the above holds true
Ex: Unrealistic: CF Period and return/risk of CF equal to TBond 30y
CFE 12m = 400 USD ( this is after costs and taxes)
DR = 3% (per year return of the US Bond 30 years. Clearly unrealistic as the risk is much higher)
Period = forever (unrealistic)
Value = 400/3% = 13,333 USD
Ex: More Realistic?
CFE 12m = 400 USD
DR = 25% (20% more than investing in SP500)
Period = 10 years ( after 10 years non CF provided. Domain has lost all the value)
Value = 400/(1,25) + 400/(1,25)^2 + ... + 400/(1,25)^10 = 1428
You can than infer the multiplier
Revenue * Rev.Multi = 1428
Rev.Multi = 1428/500 = 2.8
In essence, you need to think abt how long the domain will provide the CF and what is the risk of this CF. More negotiation than sceince indeed.