I want to say the following:
过了这几十年，中国的经济发展了很多，但是 ［at the expense of]中国的环境。
I'm not sure if the last part of the sentence is how I would say "at the expense of China's environment"; it was more of a literal translation.
Could someone please help me correctly translate this?
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。
What we often hear nowadays in the news reports or critics is that “这是以破坏/牺牲环境为代价的”. Hence you can see 'at the expense / cost of' can be translated into '以...为代价'.
Replace the original with the following sentence:
..., 可是中國的環境卻付出了很高的代價
這幾十年來, 中國經濟長足增長, 卻為此犧牲了自然生態.
If you want to maintain the order of the clauses, you can add "為此" to emphasize/highlight the relation of the economy and the environment.
