I'm a bit scared to think of how much the repair costs might be.
I'm frightened to think of how much money it will cost me to fix all this damage.
この場合、次のような言い方ができますよ。
ーI'm a bit scared to think of how much the repair costs might be.
「補修費用がいくらになるか考えるとちょっと怖い」
how much the repair costs might be で「補修費用がいくらになるだろうか」
ーI'm frightened to think of how much money it will cost me to fix all this damage.
「損傷を修理するのにいくらお金がかかるか考えると怖い」
how much money it will cost me to fix ... で「…を修理するのにいくらお金がかかるだろうか」
ご参考まで！
カナダ 
