The bottom line in Jets, like so many other things in this world, is you get what you pay for.
Having said that, there are some incredibly inexpensive jets available now, but they are only inexpensive in comparison to other jets.
If you are trying to build a jet for $600 or less you will have a hard time of it.
Probably an enforcer with fan would do it, perhaps building a skyburner from plans would as well. But you've mentioned you don't really want to build.

What I would suggest, is keep saving, it took me a long time to get a jet, when I finally did make the switch to turbines I did it by selling years worth of accumulated RC stuff. There were less expensive engines, airframes, and servo's I could have bought, but I doubt I'd have been successful.

When you see a jet being sold dirt cheap, you have to ask yourself, why is it being sold dirt cheap? They aren't cheap to start with.

If you're talking jet or turbo prop, (turbines) I'm sorry to say there is no chance you will do one for $600.
If it is what you TRULY want to be serious about, and do, then start saving now, even if it takes you a year, 2 years, whatever, you will be happier in the long run.

by way of comparrison, here is something else to consider.

I play guitar, all my guitar playing life, I had wanted an Eventide Harmonizer (an effects unit for guitar) but they cost about $3000. I bought every damn harmonizer on the market that cost less than the eventide trying to find one that I was happy with. Well, after 5 years of doing that, I'd spent a combined total of $8000 on cheaper ones, and still didn't have something I was happy with. Why? because I was impatient. Today, I have the Eventide unit and it is soooooooo worth the money. Had I only waited, I could have had one years before I got mine, if I'd just saved the money for a year or two.


Same with Jets, you can start with a Kyosho DF, or something less expensive, but will you really be happy?
Something else to consider, even if you do successfully start with an inexpensive jet, it's still going to be a considerable up front cost, if you love it, you are going to want to upgrade at some point, if you hate it, you will want to sell it, if you buy really cheap stuff, you will have a hard time of selling it. If you buy good, then when you want to upgrade, you will get more towards your next jet, if you want out, you will get more of your investment back.


As I said before, it really is a case of "you get what you pay for"