If you have 10 skill ranks that's +30 (ignoring category ranks which don't specialize). If you buy an 11th rank that would be +32 (don't forget diminishing returns).
If you specialize, you've spent as many DP as 11 ranks, but your unspecialized skill rank bonus is +30 and your specialized bonus is +50. So, you got a +18 bonus relative to what you would have had with the same DP cost, and -2 to everything outside the specialization. I think +18 is ok if the specialization is meaningful, it has to be narrow enough that it's actually leaving something out.
Diminishing returns are very important for keeping specializations balanced, because x2 ranks is not at all the same as a x2 bonus.