The thing is, you have to look at it how an investor would see it.
An investor with no knowledge of website metrics or what they mean is going to want to know one thing - money. If he gives you R100k for it, how long will it be until he makes his R100k back? What is the risk he never gets his R100k back?
If you can't prove how long it will take him to make his money back, then he needs to evaluate the web metrics including unique page visitors per month and all that stuff. Thing is, if he doesn't understand it, he can't accurately gauge the risk he is taking.
And if he does understand what all of those metrics mean, he then has to compute from them how long it will take for the website to start generating revenue, and how it would go about doing so. And if it did start to generate revenue, how much would it be able to make?
It might mean that he has to invest in an advertising campaign in order to start bringing visitors to the site. It might mean he would have to upgrade the web server to cope with the increased demand, as a silly example.
I don't know enough about business valuations to say what the value of a customer base is, but I'll say this.
The valuation of the business will directly depend on what you can prove the business earns. Historical revenue, in other words. Things like customer base and monthly website visits are indications of future financial performance, but not guarantees. For this reason, if any adjustment is made for them, it will be considered a goodwill payment (which is not unusual at all).
Basically, I would probably do the following:
"Based on historical revenue for the last 5 years, we expect revenue for the next year to X and therefore, the business to be valued at X * Y. However, our extensive customer base leads us to expect that revenues will increase, and for that reason, we recommend the business to be valued at X * Y + Z."
If you can construct a trend of how your customer base has recently increased but revenue hasn't followed yet, that will also help.
