Give tech trading a cost

**What do you want to see?**
Tech trading shouldn't give you the tech for free. Instead it should reduce its cost (say, by 75%). This cost should be moddable.

**How would it help players?**
Make tech trading not such a brain dead obvious choice, allowing retained diversity of empire tech

**Alternatives**
???

**Screenshots and mockups**
N/A

**Things to consider**
Should trading for the same tech with multiple empires reduce its cost further? If so, maybe reduce savings to 50%.

Should the savings be moddable globally or on a per tech basis? Maybe a formula in the context of the tech?

**Related issues**
N/A