There might be online printers that post letterhead printing prices up. That should give you an idea of what something like that would cost.
If the client is serious about this, get a quote from a professional printer. But please only do this if you are serious. Printers don't enjoy spending time on tyre kickers. You request too many quotes from one guy and no real jobs, they'll eventually stop returning your calls.
Oh yes, Jeff's mention about a second sheet is a good one. We call them follow on sheets here which is like the letterhead, but a simplified version, usually with just the logo or a partial logo and no contact details. This is to avoid the print taking too much space on subsequent pages. If the client often writes multi-page pieces like contracts or agreements, it is a good idea to have follow on pages.
It is more fun to talk with someone who doesn't use long, difficult words but rather short, easy words like "What about lunch?" – Winnie the Pooh