Usually when we purchase other companies, we have a lump sum that we pay the company that we purchase, and then when we assume their clients, they continue with the existing companies pricing plans etc until a specified time in which they either merge into our business for more value services, or they seek other hosting. Eventually (usually within a 1 year period) all clients are either moved over to OUR plans, or we no longer provide hosting services to them. Aquiring new clients through this manner can be good and bad, but for the most part, it's good for both the customer & the business.
It's definitely a headache trying to get everything squared away, and depending on the size of the business you purchase it can leave a big hole in your pocket. You're purchasing a company "gambling" that they will continue to stay with you.
The days of buying a hosting company for 5 years worth of their value are long gone. Even getting 12 months revenue for a sale is a tough thing to do these days. Most times I'm seeing companies sold for 6 months revenue. Sometimes even less.