How to say "at the expensive/cost of"?
I disagree with trying to translate English idioms or sentence structures word for word into Chinese. I would suggest that something like this would capture your intended meaning in a more natural way:

这几十年来，中国的经济发展了很多，但却牺牲了自然环境。

This suggests that the environment was collateral damage. If you wanted to put more emphasis on the extent of environmental destruction as the source of growth:

这几十年来，中国牺牲了生态环境，换取经济的快速发展。