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 - $1.00
Lightly Played - $0.90
Played - $0.70
Heavily Played - $0.60
Damaged - $0.50
Near Mint Non-English - $1.00
Lightly Played Non-English - $0.90
Played Non-English - $0.70
Heavily Played Non-English - $0.60
Damaged Non-English - $0.50