If you can get 15%, well 51% is only about 3.4 times as hard.
Check your math-- to get 51% you'll need to dedicate six times as much hardware to hashing as it takes to get 15%:
The formula for what fraction of existing hashing power you need add to get X% of the network is:
Code:
H = X / (1-X)
To get 15%, you need to add 17.6%. E.g. if existing hashing power is 100, then you add 17.6 more and your fraction is (17.6/117.6) = 0.15.
To get 51%, you need to add 104%. E.g. if existing hashing power is 100, then you add 104, then your fraction is (104/204) = 0.51.
104 is six times as much as 17.6.