Let’s find the breakeven point. The expected value of placing the 6 or 8 is [(5/11)*7 + (6/11)*-6]/6 = -(1/11)/6 = -1.52%.
Let b be the buy bet. The expected value is [(1/3)*(2b-1) + (2/3)*-b] / b = (-1/3)/b
Equating the two bets:
-1/66 = (-1/3)/b
3b = 66
b = 22
So, at a bet of $22 the odds are the same. The odds are better on the buy bet for bets of $23 to $39.