It depends on the price of the web hosting. If they are high value, you are obviously going to need to pay more for them then if they are all low-cost hosting packages. Also, it depends if they are transferring hardware, domains or other assets to you.
To retain the customers you need to make the xfer seamless for them - there should be almost no downtime and they should not need to change control panels etc.
I'd also factor in a value on the customer quality, based in part on how long they've been with the host you're buying from. I'd pay a whole lot more for a client who has been with a host for the last 3 years than I will for one that's only been there 3 months. Also, what kind of support loads are coming with these clients? Do they refer others to the service?
Really, there's far too many variables involved to set any kind of honest matrix on the value of a client or clients. All you can do is realize that you're making an investment in your business with the purchase, so like any business investment--get as much information as you can on the deal before hand and determine what it's worth to you in the long term against the amount of risk you assess with it.
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