As you know you can demand a tech or gift a tech but there is no straight trade (except counters from AI when you demand sometimes).
The first reason behind this is that Horizon has a "conversational diplomacy system". It's different than "trading diplomacy system" that is most common nowadays with other 4x games.
Yes, I am aware about difference, of course. But "normal" trading is still possible in Space Empires or Master of Magic style: you are making offer/ask for trade and receive "yes/no/reconsider" answer with possible consequences. I never seen mentioned AI counters, but I believe it's exactly what I want. (And in that case I am deceived with name of "demand". Is it really trade?)
The second reason is that it's easy to abuse a trade system (as in your example), each tech would have to have an intrinsic value but it's not enough, the A.I has to also consider circumstances and other factors for a good trade system. Then there is the added complexity of tech levels, unless we ignored that and only allowed exchanging undiscovered technology.
Well, while I am agree that it's not straightforward, it still goes down to the single number/pair of numbers, no? You are _already_ have needed system, in form of demand - and considered number is threat instead trade value, I suppose.
So, I think, you can safely implement similar system for trading, that would work like this:
I. Ask for trade.
1. You are asking for system/tech/money.
2. AI takes absolute value of it (monetary equivalent fits best here, I believe), then apply needed modifications. I can think about diplomacy (like, 1.1x for best relations, 10x for worst-but-not-hate), relative tech level for tech, distance from homeworld, other opponents/own planets in the system and quality for planet. But you know mechanics better, so can tune it better too.
3.1. For tech it looks suited to ask for tech in result. With possible money addition to even the deal.
3.2. For planet it can be other planet, tech or, well, money. (I am not totally sure how would it work with planets - if AI always will consider the same planet - or, for change, asks for something useless in the deeps of space, far from their homeworld, it can be quite frustrating.)
3.3. For money it's the same as for planet, but I believe, its outcome is not so important as for planet trade - anyway, if you asking for too much, be prepare to pay correspondent price.
Of course, best possible implication could be if you can add your own share of deal to the plea - but I am not very sure if it can be implemented easy enough. If not - then not.
II. Offer item (tech or planet).
Same rules as for trade, only here AI considers your stake's absolute/relative value and offers their goods accordingly. Also, I believe, planets are better to exclude as pay (but not as offer!) here, to avoid complications.
I believe, you have most of this here, so only problem is tuning needed modifiers to not allow computer be too dumb in trade, or unreasonably stingy. But I can be wrong, of course.