exact cost of database round trip vs. wcf service invocation

exact cost of database round trip vs. wcf service invocation

I've been googling for exact figures on how expensive round trips are, but
could not find any. Are there any concrete numbers given anywhere?
So, we are developing an enterprise solution. We plan to have a thin
client connected to a wcf service. In the service we plan to have an ORM
(most likely NH) with a 2nd level cache.
Obviously there are numerous benefits to having a service, prominently to
encapsulate all the business rules, enable multiple clients to manage
concurrency, etc. But! Performance-wise how does invoking the service from
the thin client compare to simply executing raw sql on the client? The
service is an added over-head and will surely be slower if the service's
orm cache is not hit. Is there some article discussing this in some depth?