If I think Greenwood can average 108 at 500k, Steevns at 105 and Griffen at 110, which one of them is the best option?
What average do you rate as keeper status? If you're spending over $500k at the start i'd be aiming for a keeper otherwise you're locking in a trade as well at some point. For me i'd say 110+ is minimum for a mid keeper, so from the examples you've listed that'd be Griffen (if you believe he can hit 110).
Griffen - 110 - $530,700
Greenwood - 108 - $506,100
Steven - 105 - $455,900
It would seem that Griffen has Greenwood covered, under your assessment. It's unlikely that the $24,600 loose change will make a significant change to your other selections. Possible, but unlikely. Griffen also has the advantage of the Rnd 13 bye, which really gets him home over Greenwood, and gives him a tick over Steven as well, as they both have the "crowded" Rnd 12 bye. So now it comes down to Griffen Vs Steven. The answer comes down to 2 things/questions:
Are you happy to carry Steven through the season as an M8 (maybe if disaster strikes M7), if he averages 105?
Can you make a significant improvement somewhere with the $74,800 loose change Steven gives you over Griffen?
Keeping in mind, there is obviously a margin for error in your expectations, which favours Griffen. If they both come up 5 short of your expectations, Griffen can fill a F7/8 role, where as Steven will just be leaking points to the better teams.
I would think the answer is this:
Does using the $74,800 to upgrade a player somewhere else, give you more confidence in that new player succeeding over the old player you had, than you have confidence in Griffen succeeding over Steven? It might for example upgrade a slightly risky Midpriced Def to a more "stable" Premium option in that line. In simple terms go with the combination you have more faith in:
Griffen + lower priced selection
Steven + higher priced selection.
If you can't improve your confidence with the loose change, go Griffen.