Pittsburgh didn't trade for that pick. However, the 49ers did just trade two 5th round picks for the Jets' 4th round pick. And that's straight up-- as in, they didn't even give up their own 4th. And the Jets also traded away a 4th round pick for next year's 3rd from the Lions. Again, straight up. That's a normal trade value for any draft.
So your assessment of trade value is way off. It wouldn't have taken much to move up or even acquire an extra pick this round.
And there have been 7 trades this round with teams moving up or acquiring picks so there's no shortage of partners either.