[WraithLord]

Based on my own personal experience with EDM summons I'd say that Grendelkins are well priced for their value.
A thing to take into acct (that was mentioned here) is that EDM summons tend to have some roles in which the excel and others in which they sub-perform. So when weighing their price it's important to amortize all the aspects, good and bad, weak and strong points. You can't say that X is OP just b/c you had the bad luck to meet it when it's on "upper ground" (or favorable conditions).