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If you want an assisted flipper, pick your favorite Kershaw and game over. I put Kershaw ahead of anything by CRKT because of the service and support KAI offers on all their products. Kershaw offers the Knockout, Camber, Piston (if you can still find it) and Link along with all the various Ken Onion designs. If you're craving a little "Hinder'ish" action, Kershaw has the budget Cryo and Thermite, both Rick Hinderer designs. Many of the cheaper, Chinese manufactured Kershaws are fine knives. I've been particularly impressed with the new Valmara. Kershaw also has the ubiquitous Skyline as a manual flipper that makes a great all-around EDC knife.
If you want a manual flipper (other than the Skyline), your best options are something from the Kizer Vanguard series. I have several of these knives and they're each really well done. Probably the BEST value knife in the $50-100 range offered at this time. The new Kesmec looks sweet and I can personally vouch for the Gemini, Dorado, Dukes and Kyre. Kizer used to struggle with weakish detents on some knives but every Vanguard knife I've purchased has been spot on for detent strength.
A close runner-up in manual flippers would be a couple of models by RealSteel; the H5 Gerfalcon and the E771 Sea Eagle. Avoid the E571, its not a great flipper.
Buck MIGHT be an option. I've heard good things about the Marksman, although I've never handled one. A lot of people will recommend the Buck Vantage but I can only go along with that if you have a way to hand pick your knife. The quality, fit and finish on the Vantage knives range from acceptable to horrible.
Spyderco offers a few flippers but they're all out of your price range. Honestly, I've yet to find a Spyderco flipper that was a really great flipper. The classic Spydie leaf shape blade doesn't have a great weight distribution to flip terribly well.
You might take a peek at the new Stedemon ZKC (Zon Knife Company), it's a big blade but is roughly close to Kizer Vanguard in materials and quality.
The Boker Urban Trapper is a decent knife. It doesn't really "fire" out as a flipper, but it's acceptable. This is a great option if you need a slim, disappearing knife in your pocket.
Probably the closest you can come to a genuine feeling Hinderer is to cruise the Exchange for a used ZT 0561, ZT 0562 or a ZT 0566. You could add a few dollars to the top of your budget and be in the market for a good, used 0566 which work fine assisted or de-assisted. The 0561 and 0562 will take more dollah.
You might even be able to find a used Benchmade Precinct for right around the top of your budget...although I have no firsthand experience with that knife, Benchmade is known for quality and service.
Good luck and have fun!
I'd also say this about many Spyderco knives. I know they're very good quality but I just don't like the spidey-hole.
No one has mentioned the Kershaw Strobe, but its a fantastic flipper for the price (a bit under $40). Slices well and isnt bad in the hand.
Buck MIGHT be an option. I've heard good things about the Marksman, although I've never handled one. A lot of people will recommend the Buck Vantage but I can only go along with that if you have a way to hand pick your knife. The quality, fit and finish on the Vantage knives range from acceptable to horrible.
At this point it's a shame you can't handle all three before you make a decision. There may be one you prefer due to ergonomic considerations. I only own the first two, but can offer a few thoughts:
I'd have to call the blade shapes and steels equal with these knives. Prepare yourself to learn how to sharpen and maintain an edge if you don't already know how. VG-10, 14C28N and 440C are great general use steels, but will tend to give up the edge fairly quickly. Fortunately, all three are fairly easy to sharpen. You'll greatly improve your level of satisfaction with any of these knives if you can maintain the cutting edge.
I can say that the G10 scales on the Kizer have the most aggressive texture of the three, if that's a consideration. The Stedemon looks the smoothest, but has some texture milled in. The Sea Eagle has good ergos but probably the least "grip". You can feel the steel liners when holding, if that makes a difference. The Kizer has a more "hand filling" grip than the RealSteel, and the Stedemon looks like it might have the best ergos of the three.
The Intrepid is a budget version of a knife by designer Ray Laconico, the other two are in-house designs. Some people like designer names on their knives and jeans...
All things considered, you've picked three knives of very equal material quality and likely performance. I don't have any first hand experience with the Stedemon but Gary Graley, whom I respect as a reviewer, spoke very highly of it http://www.bladeforums.com/forums/showthread.php/1400828-Stedemon-D-01-folder. Stedemon does not make the ZKC; they're made for them by Zon Knife Company. This is a new name for me so I can't vouch for them from personal experience.
I don't think you can go wrong with any of the three, but if I had to choose I'd probably give the edge to the Kizer, but only from a standpoint of service and support. None of these knives come with the awesome customer support that Kershaw, Spyderco or Benchmade can offer, but Kizer has set up a Stateside service center that can handle some routine things like screws, washers and pocket clips. They also tend to be the most active within the BF community.
I think you'll be happy with whichever knife you end up with. Be warned though: the knife hobby is a harsh mistress. The knife you buy today is the knife you'll use to open the NEXT knife you buy!