A reasonable price could range from 4500 to 5500
There are a few 600/4 APO HS G's currently listed on evilBay within the price range above, but taking into account fees, I would think even $4000.00 is possible especially if by private treaty. I know people buy from Craigslist and charity shops, so both of these are options as is Amazon and retailers like KEH and B&H Photo, but I would also post a WTB ad on Dyxum stating what you want and how much you're prepared to pay and see what happens. It's cost nothing to ask and you'll know that it's been owned and used by a dedicated shooter and likely well cared for.
If there are any free online classified sites, check these in addition to any used lens retailers that may offer items on behalf of the owner and you should make a low-ball offer and see what happens. The owner may need the funds urgently, so you may be able to score a copy for a lot less than the 'going rate'. After all, 2x Min 300/2.8 APO lenses were sold for GBP £450.00 and GBP £750.00 within a week on evilBay (UK). Whoever bought them now own one of Minolta's benchmark tele-prime lenses for 75% and around 50% below the 'going rate' and both were more-or-less complete in their alloy cases, filters, strap. keys etc, and those prices are unheard of, so I believe the buyers will stand to make a fair chunk of change when re-sold, so such things clearly do happen, and it may do so for you too as your not talking about a $100.00 SAL50F18 DT prime, but the cream of Minolta's lens back catalogue and some serious money, though you need to be prepared to ask relevant questions about any internal ingress, lens coating de-lamination and others, but if you can try before you buy, then that would be very handy, and depending from who/where you buy from, I would make payment using a credit card and/or PayPal for payment protection. I would also get info about Minolta lens specialists should you need it cleaning or servicing, but these lenses are usually bomb proof.
In addition, if you're a member of Flickr, take a look at
this and maybe post a WTB message there.
I wish you well with your quest.