IMO, the vintage Gerber Mark I is the best boot knife ever made.
You can still buy them in "like new" condition on EB for less than $200, if you're patient. One w/a leather sheath and a low serial # sold for only $175 recently. The following photo is of that knife and sheath.
And if you want to buy a user in lesser condition, you can get one for even less. In fact, one such example just sold on the BF Exchange for just $50.
I bought a Spyderco Nightstick when it was released but quickly sold it because it didn't compare well w/either of the 2 Gerber Mark I's that I own. BTW, the Nightstick is only sharpened on one edge, while both edges of the Gerber Mark I are sharpened.
I also just purchased (but have yet to receive) a vintage Kershaw Special Agent which is similiar to the Gerber and was designed by Ichiro Hattori -- a legendary Japanese knife designer who designed Kershaw's very early Hunter Series, the Trooper and the Special Agent.
The Special Agent is doubled edged and can also be had on EB for less than $200. The photo is of the one I bought for that price but, like the Gerber, you can buy a "user" in lesser condition for much less.
For reference, see:
http://hattoricollector.com/2015/09/24/kershaw-trooper-and-other-hattori-daggers/