Many variables would need to be considered.
If it didn't have any organic signups, then at a very basic ballpark figure I'd be going for around 6 months gross profit.
If it had organic signups, then I'd be looking at maybe closer to 9 months.
But that is a really basic way at looking at it, in practice I'd do a forecast on Excel to see what I'd expect the future cashflows and do various sensitivity analysis / what if scenarios to place a true value for me of the business.
Adding a little detail, I'd:
Estimate the next 12 months commission based on me spending only the bare minimum time (replying to emails if it's that type of affiliate business). This would include worst case and best case scenarios e.g. worst case would be commission falls by around 20% per month, best case commission falls by around 5-10% per month
- put a $ time value on the time spent replying to emails etc, effectively my opportunity cost
12 month commission less time value gives me the profit I'll make over 12 months.
12 month is just a figure I feel I could reasonably aim to forecast, anything past 12 months I'd consider bonus money. Also, I'd expect anything past 12 months to be very low, given I am assuming I wouldn't be spending any time promoting etc.
So this gives me a range from best to worst case profit I'd make.
I then compare this to the price I'm being offered and decide if there is value in it for me.
Note as part of the best/worst case scenario, I'd be looking at things such as number of players, which networks the players are on, to help me gauge the variance I could be facing. As we all know, 10 players on a small microgaming skin generating $2k commission/month is worth far less than 100 players mixed across 888 and party on rev share deals generating $2k commission per month.
I'd then also review the opportunities available - what skills do I have that could grow the business that the current owner doesn't have, or was he lazy? I'd then estimate the potential to grow the business, and the associated commission I'd gain from this, vs the costs + time costs of growing the business.
So overall, I'd perform to reviews:
1) Decide if it's profitable to just spend the minimum time and receive a monthly commission payment, based on the price I am paying upfront + minor time costs + any other costs such as hosting etc.
2) Even if 1) is or is not profitable, I'd also look at whether devoting time and money into the site could lead to a profitable site for me, based on the upfront cost + ongoing marketing/development and associated time costs.
Also one of the most important factors is to find out in detail why they are selling and do some research on the seller and their business to estimate the risks of being scammed.
Just my thoughts