As a lefty collector of gent's knives I can't add much to the list, but I can give you some thoughts in the price range you are looking for.
The Mnandi is a good choice and is available as a lefty.
Moki knives can be exquisite and are all lockbacks, so handedness is a non-issue there. In your price range I would try and hunt around for a vintage, more exotic pearl inlayed version. The new ones they are selling in the U.S. are nice, but the older ones are really prime if you can find them.
Wm. Henry knives are really ambidextrous with the buttonlock and "reversible" thumbstud. I put reversible in quotes because they use BLACK loctite on the threads of the stud and even with the proper sized Torx bit (T5 and smaller) I have yet to remove and flip one without buggering up the screw, and I've done 5 of these. You CAN get a replacement screw from Wm. Henry if you call and ask nicely.
Customs- You can get some nice custom slippies in your price range or even some lockback, but $500 will get you basic steel and MOP, not much more. The mid-upper level makers don't even really start until you hit $750 (Steigerwalt, Osborne, Sawby). I would look at Polk, Montjoy, Ray Cover Sr. in your price range if you want to go the custom route, which would put you around $400 at most. There is sort of a void in the $500 price range. I would look atwww.knifeart.com for the be selection in that price area. One thing I almost forgot, the South African makers make some AWESOME ambi knives with VERY low profile flippers in that price range (I have quite a few of those), but they have been moving toward thumbstud openers since they are more popular in the U.S. Andre van Heerden makes some that are especially nice and in your price range. They are ALMOST always linerlocks.
www.bladegallery.com has a pretty good representation of what is available from the S.A. makers, as does knifeart.