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.”
NM-Mint - $0.80
Lightly Played - $0.70
Moderately Played - $0.70
Heavily Played - $0.60
Damaged - $0.50
NM-Mint Non English - $0.80
Lightly Played Non English - $0.70
Moderately Played Non English - $0.70
Heavily Played Non English - $0.60
Damaged Non English - $0.50