Don't confuse Jackson's defection with Grundy's value, two separate issues, I'm looking at each trade in isolation and weighing up the value.
1. Grundy (pick 27 & 300k salary)
Outstanding value, particularly if the sub gets scrapped for an extra player
2. Mitchell (pick 41 & 50 & part salary)
Pretty good bet, low ball risk in a draft with potentially 50 selections.
These were the classic Moneyball trades, both players still have good football left in them & could easily finish top 5 in the B&F. Both Collingwood & Melbourne fair weight for top 4 so a premiership would vindicate these additions. In the case of Melbourne they haven't necessarily had a net gain, more stabilising the list but they improved their draft hand this year & next.