There are many factors to consider.
The main one I would consider is the renewal rate of the clients.

There are lots of fly by night operations out there that promise the world and gain dozens or even hundreds of clients but have no retention rate.

I would say that, depending on the client retention rate, a hosting company is worth 3 months to 6 months gross revenue.

That is just for clients. Other things such as software, websites, domain etc. have to be taken into consideration as well.