Interesting....
In general, by mid-game when I'm not longer starved for cash I completely move all of my trade to City States. I see no reason to give opposing Civs any more money and if I don't really NEED the money, I'm not losing much.
I think the Science 'trade' REALLY needs to ramps up in order to encourage inter-Civ trade a bit more. For example, in my last game, I was making 439 Science/turn at one point. And trade routes with other Civs would net me 1-3 more depending on how many techs they had that I didn't etc. Really? 1-3? Why bother?
On the other hand, in the EARLY game, those bonus few science points are a real help. If you're only making 20-30 since, getting 1-3 more on a few routes is very handy indeed.
So, IMO, the science should scale up through the game to keep things interesting. If I'm making in the 400s, the bonuses should probably be at least 10-15 to be meaningful.