The Delta website offers a comparison of the value of FF miles and dollars.
A general member can renew a Sky Club membership for $450 or 70,000 miles. That calculates to .64 cents per FF mile. For a Silver Medallion, it is $350 or 50,000 miles -- .70 cents per mile. Gold and Platinum come to .67 and .75 cents per mile.

When it comes to Sky Club memberships, Delta seems to put a rather low value on FF miles. (Well under a penny a mile.)