I can see valuing a colony-less system [base price, scaled by distance + some amount for resources in system] as possible (debatable, as closed borders cheese is a thing and access for ships is value all unto its own) but how would you price something like an alloy world, or even the science output of a ring world?
It's better to overvalue a planet than to undervalue a planet, but it's totally possible to value a planet.
A pop costs 500 food to grow. (or 100 alloys for robots, so 600 value if robot)
A planet is worth 500 minerals per district (not really, but lets say). And 500 minerals per building plus 500 minerals per upgrade on buildings.
A planet costs 200 food, 200 consumer goods and 200 alloys to colonize too so 1200 value.
A planets value comes from it net production. I'd say current net production x 120. So what it would produce in 10 years if nothing changes.
So the cost of one planet is 1200 + organic pops x 500 + robotic pops x 600 + districts x 500 + 500 per building or upgrade. That's JUST the planet.
The system itself costs 75 influence and 100 alloys to build, that's already 400 value and 75 influence. Influence is hard to evaluate. But I think we can bump up the cost to 2000 value total easily, or even 3000 just to be on the safe side.
For the space resources I'd simply say the (potential) production x 120, so 10 years worth of production.
An AI should NEVER trade a system that would cut off one part of their empire for another. An AI should NEVER trade a system with a megastructure (ruined or otherwise), including habitats and gateways.
So an example system. Has 3 energy on the star and 6 minerals on two asteroids. (3 + 6) * 120 + 3000 = 4080 energy (or equivalent) for the AI to agree to sell. That is assuming the AI is OK with a "fair" price.
If the system also has one planet. The planet only has 2 mining districts and 4 pops. It produces 8 minerals, costs 4 energy, 1 consumer product (let's just say).
The net production of the planet is 8 - 4 - 1 * 2 = 2, and 2 x 120 = 240.
The mining districts are valued at 500 each, so 1000. And it costed 1200 to colonize it. So 240 + 1000 + 1200 = 2440 for the crappy little planet.
Giving a grand total of
6540 value for the system.
It would be possible to buy fringe systems and a few underdeveloped planets, any developed planet would be vastly too expensive. And that's
fine. It's GOOD IMO.