Obviously it is better to produce a cheap 6 win player yourself, but that isn't what we are discussing.
If we were comparing two 6 win players, the cheaper one would be a better value. But again, that isn't what we are discussing either.
We are comparing a 4 win player to a 6 win player, and you are arguing that the 4 win player is "worth more" because his $/win value is lower. It I my opinion that being able to concentrate 6 wins into a single player is worth more than having a 4 win player and another 2 win player because you don't have to use up 2 roster spots to get those 6 wins.
I guess that was my long way of saying that if Wren can get Tulo for Simmons and make the money work somehow (I seriously doubt he could), then that's a trade he needs to make without hesitation.