I paid about $250 for a MSI 1080 with liquid cooler in November, and $190 for a Zotac 1070 earlier in 2020. Both of these were from ebay. I was able to get some for even less on facebook (like a oem 1080 for $175 in december) In my opinion, they they should cost that amount at most in 2021. These cards are like half a decade old and nowhere near the performance of modern flagships. 1080ti should be <$400 IMO.
HOWEVER, the shortage has made these prices inflated, so you are unlikely to get one for what they "should" cost.
Video cards are "worth" as much as people are willing to pay for them. If you want to pay $300 for it, go ahead. If you want to wait until prices return to normal, do that.