With the trade score of the WR so high, trading for just a first was unlikely.
Even then, there's no guarantee that the draft pick will bloom.
While negotiating with him, I'm sure 500 has thought about it and he probably sees that at that position of the draft, this was far more the better value for his team than through the draft.
If I could've given an additional pick or player, I would have as well. Trade meter is a good suggestion, but handicaps are built in it as well.