I run a small web-hosting ISP, I've been approached by another ISP who
wants to focus on another part of his business, and wants to sell me
approximately 120 of his hosting clients.  Is there a rule of thumb
for how much to pay for such transactions?  I can figure my own cost
of customer aquisition, but i'm curious if anyone has stats on what
ISPs pay to buy accounts in bulk.  Or if there is a rough
rule-of-thumb of any sort to use.

Using your cost of customer acquisition as a rule of thumb is a good
starting point (any service business would use the acquisition cost as
a metric for this).

One would need to know more information to get a better idea of what
the value of these customers are:
-How long have the customers been with the ISP?
-What features/services are they used to with their current ISP that
they will no longer have under your hosting environment (anti-SPAM,
web-email, etc.)
-Do they pay their invoices on time (ie: are any of them "Problem customers")?
-What pains will each of the customers experience during the
transition to the new ISP (ie: will there be any customer loss during
the transition).
-Does the other ISP have any "brand recognition" in the industry that
has generated "Goodwill" with the customer base?  Marketing efforts
that may be of value?

Thanks - we aren't buying their business - they want to keep their
"brand" but focus on the non-hosting aspects of their business, so I
don't think