If you're not racing and even if you are, there are lots of new frames and forks which can be had for under $600. There's someone selling new CAAD5's for $500, including carbon fork.
It also depends on what you expect from the frame. Performance, zoot factor or just functionality?
I bought a Coppi Gavia, new, which is straight guage aluminum for $75, plus an ITM carbon fork with alloy steerer for $100 and I was in business. Not the lightest but I like the geometry and most importantly, it fits. I ride it 100+ miles/week and it's highly functional. I've got a Vortex and a C-40 but since December, the bike that's getting the use is the Coppi. When it's cold, wet and you're grinding into a 20mph headwind, I don't spend a lot of time thinking about the frame. Set a price and get what fits.
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