This is a really weird feature of the game’s meta that comes up a lot: most players do not sell tech at a premium.
It is VERY common for non-ally players to swap tech, not always on a like-for-like basis, and to then pay for any difference in transfer cost - e.g. banking 3 for weapons 2 + $15.
The most common thing in the meta is get a set of allies, and share tech at cost of transfer with them. This is one of the main benefits of being in an alliance. Sometimes in very large alliances smaller players may send cash to purchase extra science to the larger ones - as for the most part 3 players with 50 science is better than 6 with 25 - but this isn’t usually on a tech-by-tech basis.
But selling tech at a premium is very uncommon and I have never actually managed to convince a player to purchase at a premium.
Putting people’s irrationality aside…
When considering selling tech, I always use infrastructure costs as a rough guide. If i’m selling you something then I’d better get a benefit out of it - if I can’t build something out of it then what was the point? So the cost of a science point, or 2-3 econ always feels like a reasonable premium to me.
When buying it, it would depend on the value of the tech. Banking is the most obvious as it is directly comparable to economy: 1 banking level is equivalent to 7.5 economy.
So $100 for banking 2 is actually a pretty good deal for both parties. 7 economy is typically around $210, so buying banking at that value gets you over double the boost you would have had from just building econ (ignoring the extra benefit that you can’t lose it). At at $30 + $70, the seller is likely getting an additional 2-3 econ. Your gain is actually bigger for the purchase than the seller is getting.
Selling a tech for essentially $3 is not worth it. You can do nothing with $3, and the benefit THEY will get from that terraforming level is about a 15% boost to spending power - so at that level probably it’s equivalent to about $50 for the next production. Selling it at a rate that would get you an additional 2 econ would be reasonable.
Having said all of this… there is no usual rate so that’s bullshit. Almost no one ever buys or sells tech at a premium, although I really believe - especially in games where trade scan is turned off - that this is because people are irrational.