It is a balance between what you are willing to pay and what the buyer is willing to accept. Even expensive game codes have been sold for as little as 5k, but that is rare, the most I have heard of was about 20k in BTBS but some people will accept collectibles and if you calculate their worth against Lassy guide that could make the price a lot more if expensive collectibles are traded.
Usually you need at least 10k in pure tbt to get a bite if it's a 40$ game but it depends entirely on the seller of course. Last few threads I've seen selling codes always included the line "10k is the lowest I'll consider, doesn't mean I'll accept it though" or some variation. sometimes they'll take collectibles too which makes it a bit easier to buff up your offer.