It's cheaper to buy ... at a store than it is to buy ... at Amazon.
Buying ... at a store is cheaper than buying ... on Amazon.
ーIt's cheaper to buy those at a store than it is to buy them at Amazon.
「それらをアマゾンで買うより、店で買った方が安い。」
It's cheaper to buy ... at a store than it is to buy ... at Amazon. を使って「～で買うよりも～で買うほうが安い」と言えます。
ーBuying those at a store is cheaper than buying them on Amazon.
「それらを店で買うのは、アマゾンで買うより安い。」
Buying ... at a store is cheaper than buying ... on Amazon. を使って「～で買うよりも～で買うほうが安い」とも言えます。
ご参考まで！
カナダ 
