Why does it need to be assisted opening? I personaly think the time difference betweed a thumb stud knife and an assisted opener is negligable. There are lots of 4.5"+ folding knives for less than a hundred bucks that are capable of any knife type task, but I can't think of one with assisted opening. If you can live without that feature, I reccomend SOG's 5" bladed Pentagon Elite. Cold Steel also has some nice longer bladed knives for reasonable prices, and so does spyderco, but I don't believe any of those feature assisted opening.
If you look around, you might find a Darrel Ralph knife that falls within your parameters, but I doubt it will happen for less than 250. Other than that, I'm out of ideas.