For makers I know in the $500 neighborhood, I'd give you the following list:
RJ Martin (especially his Devastator model)
Kirby Lambert (his prices are going to go through the ROOF shortly I'd guess, and he's a good bargain right now)
Mike Obenauf (I think his Model 2 is destined to be considered a classic)
Tom Mayo (You'll have to wait on the secondary market or dealers, but his work can be had in this price range if you're patient- he's not taking orders at the moment)
Bob Dozier
Mike Draper (makes the Sabra and I bet he'd do a framelock for you)
Bob Lum (especially the Chinese- 2 year wait)
That's what I've got at the moment for a list of makers.
I'll say this about your question regarding the Sebenza and value of that extra money: it depends. If you're looking for resale value, the sebenza holds it- but not as much as a well bought Custom. By that I mean picking up someone who's going to be big (like Obenauf or especially Lambert) when their prices are still pretty reasonable. Later you can often make at least a profit on selling these should you have to. Not going to happen with a sebenza.
If you're looking for function: probably not, but then again as is stated here all the time, there are a lot of knives that are CHEAPER than the sebenza that'll do the job (cutting) just as well.
To my mind, it may not be that you're getting a substantially higher benefit by spending the extra money on the custom, but you ARE getting a custom. I'm sure you'll get a lot of people who'll tell you how functionally perfect and well executed the sebenza is. They're right. However, at the current prices you are so far out on the cost/benefit arc that it really doesn't matter.
I've owned 3 sebenzas, and often said I didn't "get" them. If that's what you want, then more power to you, its YOUR money after all. I'd rather have the custom. It'd be a personal thing, for sure, and you can get a LOT of little things done right for the money if you choose wisely.
As far as Kevin's knives, he is a GREAT maker. His folders are done very well, and he takes a lot of pride in his work- which you can really see in his finishing work.