Your Buck M9 is part of a short but very interesting history of the M9 bayonet, and depending on the specific "version" you have, could be worth quite a bit more than you have been led to believe thus far. The M9 bayonet, in its short time has seen dozens and dozend of changes, some big and some not so big. Regardless of the magnitude of the change this many variants to one pattern makes the M9 VERY collectable.
Buck became a subcontractor for the M9 bayonet in 1986. The prime contractor was a company called Phrobis. Due to some sort of dispute, Buck wound up purchasing the company along with the product rights to knives they developed. After three years the contract was terminated with Buck and was opened for bidding. A company called Lancay won the contract.
Reportedly, the Lancay units were low quality and failed in the field...mainly broken blades. As a tool these are not regarded as desirable knives but to the collector they are part of the rich history of the M9.
Sorry, got a little of track there. Your Buck M9, if it is the "civilian" model, i.e. the ricasso is stamped "Buck 188 / U.S.A" or some variant thereof, would fetch around $100. BTW, the "/" is a date code (1989) so yours could be some other symbol.
At the other end of the spectrum, an "issue" model baring the same sort of stamp as mentioned earlier with "M9 USMC" above it ( made in 1991?) could fetch, in mint condition, $500 - $600 (ref. U.S. Military Knives Bayonets and Machetes, Price Guide, Forth Edition pg.24 by Silvey, Boyd, Trzaska). Even more valuable than these are the bayonets Buck made for the USMC in 1992, having a one piece blade-tang construction, as there were only about 250 units made. Within this 250 units there are two variants, further increasing the value of each ( one had an Allen head bolt securing the latch plate to the tang within the handle, the other had the tang "rivited" to the latch plate ).
Of course there are all sorts of in-betweens, for example the Phrobis made Army issue model would be somewhat more common and correspondingly of less value. Conversly, in 1991, Austrailia purchased Buck-made M9's which were marked with the Commonwealth "Broad Arrow" proof in place of the date code. These, I suspect, would fetch something similar to the USMC M9 (1991).
Sorry for the lenth of my reply, I could go one forever. Like I said, the M9 has a short but rich history. Wanna hear more?