TBH, I'm not sure I do. If you're referring to the cost of the new unit as the $900 chance, then a.) I don't recall seeing where the $900 comes in (cost of new unit, shipped, in AUD?) and moreover b.) if it is new, it's not really taking a chance at all.
IMO your best bet is the "$200 chance". Find the cheapest seller with a sterling reputation over a long period of time, for a used unit, and cross your fingers. And although you'd really never want to have to, make sure you have an avenue for pursuing if there's a problem. Unless you just absolutely must have an unlocked unit, I'd also consider a vanilla 3770. They can be overclocked reasonably; I've done several that wound up well past 4G 24-hour stable overclocks. Thing is, after a certain point, air isn't viable cooling - and in my experience, the 3770's will get you up to that point. The performance you gain after that, while not completely insignificant, increases cost and hassle in an exponentially inverse proportion.
Of course, I do realize a 3770 is not the same as a 3770k. I have done unlocked units that I could get another 18% out of over the fastest non-k chip overclock, but this was the CPU part of an overall synthetic gaming score which is definitely influenced by GPU. In the my tests with the same GPU/same motherboard etc, I have gotten a plain 3770 to 92.5% of what I could get an unlocked chip to run stable. To me the extra 8% or so doesn't seem worth the cost and what you go through, for what you get in real 'performance perception'. Most of the people I've let test drive these systems cannot even tell the difference, and the actual impact on frame rates is - for the most part - spectacularly imperceptible.
In any case, best of luck