When I opened the envelope, I was delighted to find a check

When I opened the envelope, I was delighted to find a check ____$500. 

请问这里的介词为何填 for 表示什么呢？

for $500用作后置定语修饰check，字面意思是“代表$500”，从而引申出“等值交换”，即“check”和“$500”是等值的。for常用于表示等价（报酬、赔偿或比率关系）等的“交换”。又如：

I bought a book for ￡3. 我花3英镑买了一本书。

She gave me theirold TV for nothing. 她把他们的旧电视机送给我了。