Seems like you can get them used on eBay for around $450-500 for the S8 and $500-600 for the S8+.
Is it now a good time to buy the Galaxy S8/S8+? Saw the awesome Optus deal for the S9/S9+ but used S8 seems like even greater value when joined with a good, cheap prepaid plan.
I advise strongly not to purchase a Samsung S8. I have had mine repaired twice in the last 6 months with the same problem. The camera has a fault with the image stabilizer and does not focus unless you hit the side of the phone. This is a common fault if you google it.