As long as Thundering Mightmare is paired with another creature, each of those creatures has “Whenever an opponent casts a spell, put a +1/+1 counter on this creature.”
Near Mint - $0.70
Lightly Played - $0.70
Played - $0.50
Heavily Played - $0.50
Damaged - $0.40
Near Mint Non-English - $0.70
Lightly Played Non-English - $0.70
Played Non-English - $0.50
Heavily Played Non-English - $0.50
Damaged Non-English - $0.40