Re: Reasonable factory.204 Ruger
I have three .204s: Cooper, Remington, CZ. You'll find no stainless heavy barrel under $600, unless a Cabela's has a leftover Rem. stainless 700 SPS Varmint; you could buy a Howa barreled action heavy/stainless for about that, but even a Boyd's stock - if they make them to fit - would be at least $150 more.
A blued chrome-moly CZ 527 Varmint, plus the cost of rings; or a "regular" Remington SPS V, plus the probable cost of a decent stock later - is about as close as you'll get to 600 bucks. Savage might have something close, but it too wouldn't be both HB and stainless.