My first thought given the target price was Derringer. Why has no-one mentioned that? I have never even shot one, but it seems to me the perfect gentleman's pocket pistol, akin to buying them pocketwatches but more fun.
I looked at Bud's and for just under $150 you can get blued or stainless in 22lr, 22mag, 38spl or 9mm. I'd go for 9 or 38 and you could have the barrel engraved then put some gold filler in it. I saw this stuff the other day and really want somebody to try it and report back
http://www.forsterproducts.com/catalog.asp?prodid=700512
Of course with a firearm you'll have transfer fees to pay on top of the Bud's price but I suspect you could find a dealer to work with you locally.
Like most other comments, it's really hard to go wrong with a knife. Especially if it's a $100ish knife with some engraving done.
There's also the Leatherman MUT if they're shooters, that's a cool one. Leatherman tools can be a bit more personal, tailored to what each groomsman does for a living (or hobby).