I can buy and blueprint a 700 action for less than that. I buy used cheap 700 ADL rifles for $300-350 for donor guns. And my smith only charges me $250 to blueprint, fit and chamber a barrel, re-crown, tune the trigger (I normally do this myself), DevCon bed the action, and float the barrel. And if I'm using a factory barrel, he hand-laps it for me and cuts a recessed target crown included in the same price. Everything is single-point lathe cut, and it's very high-quality work.
I know this is not the case for everybody, and not everybody has a local smith that has great prices and exceptional turn-around time, so, I do realize that for some folks the $600 action would be the better way to go.
However, you have to know, that just because it's a custom action doesn't mean that it's not in need of a check to make sure it needs squaring up. And I would highly encourage everyone who buys a custom action have their smith check the action for squareness just to verify it doesn't still need some work before proceeding with the build.
This is also a good option, and the prices are cheaper if you get in on a group buy. They advertise them on here from time to time...
http://pacifictoolandgauge.com/ptg-...extreme_fp_assy_and_knob_color-black_anodized
Another good option for a custom action is the Curtis Axiom...
http://www.curtiscustom.com/product/axiom-4/