For people looking to move up and down in these later rounds I would like to say a word. I'm new to the game but as the rounds go down so does the talent drop off. In some cases dramatically.
Trading a 1st rounder for a 2nd and 3rd and more is good value due to the increase in talent. But as the talent decreases a 3rd rounder is no longer worth a 3rd and 4th because the upgrade in talent is so much less. Tomorrow when we hit the 4th round the talent drops off even more.
says the guy trying to trade for one of those 3rd/4th round picks*

I disagree with your point you could argue that with talent decreasing teams are more desperate to fill out their rosters with good players rather than average-to-below average ones and therefore the price --should-- be just as high.
Value continues to be relative throughout the draft. After all, if you're arguing that the talent is round 3 is lower than round 1 (which it of course is) then the 3/4 someone gets for that 3/4 is ALSO worth less than the 2/3 you said is good value.
* I don't agree the talent drops off in the 4th, but we can agree to disagree