The price of the stock I bought last week has dropped a lot.
The stock I invested in last week has gone down quite a bit since I bought it.
この場合、次のような言い方ができますよ。
ーThe price of the stock I bought last week has dropped a lot.
「先週買った株価がかなり下がっている」
to drop a lot で「かなり下がる」
ーThe stock I invested in last week has gone down quite a bit since I bought it.
「先週買った株が買って以来かなり下がった」
to go down quite a bit で「かなり下がる」とも言えます。
ご参考まで！
カナダ 
