Tech gets more expensive as you gain more of it because it represents the greater complexity later technology has compared to earlier technology. For instance, a computer costs more to build than a flint arrow might, to use an extreme example. So when a nation with a tech level of 50 sends that 50 tech to a nation with 6000 tech, it shouldn't actually make any difference to the more advanced nation's tech level. It doesn't matter how much research on flintknapping you send, it still won't be any new information to them.
I was thinking that maybe tech just represents amount of technology, but I think that'd be included in infrastructure. The description indicates it means level of technology, as well. Wouldn't it make more sense to have a system of research, instead? You can send any nation any research, but if they've already got it it won't make a difference.
This could even be represented by the current system, except without the ability for lower tech-level nations to send to higher tech-level nations and without sending research actually reducing the sending nation's level of research. There could just be a fee, say, so people don't abuse it - the more advanced the tech, the more it costs to send over (because the more advanced it is the more tools, knowledge, education, etc. you need to make use of it).
I don't know, just an idea. What do y'all think?