For any resource, if the AI has none, they will greatly value a single unit of that resource
1 unit of antimatter to a civ that has none, the credit offer is around 80. 2 units of antimatter, the offer is 90 -- so that second unit of antimatter it values at 10 (this is approximate, the maximum credit offer the AI offers for the antimatter I offer). 10 units of antimatter, the AI offers 120 ... past that first antimatter = 80 credits, this averages out to 20 credits for 8 units or two and a half credits per unit of antimatter. 50 units of antimatter, the AI offers 170 ... so the extra 40 units of antimatter the AI only offers roughly one and a quarter credits each.
This AI has no antimatter, and its fair to deduce they have no antimatter production. They could build a substantially enhanced fleet with prototype hyperdrives and antimatter weapons ... even if they favor non-missile weapons, the enhanced engines should be valuable to any ship design, but the offer for a decent stockpile seems rather pathetic.
What's worse and makes this truly terrible is that after selling them 1 antimatter for 80, in a separate offer I can sell then 1 antimatter again (after having already sold them one) and they will again offer 80 credits. This makes the AI valuation of resources truly bizarre, inconsistent and nonsensical. If I have a decent stockpile of a resource the AI has none of, the AI should value it sensibly. Its fine to have some degree of "I have no X, so I'll pay a premium to have the first unit, not as much per unit for a larger quantity, I should get a discount for buying in bulk," but its way too imbalanced at this point.