"Seventy-five hundred" is one way to say the number 7,500. You can say some numbers between 1,000 and 10,000 this way:

6,200 = sixty-two hundred

8,900 = eighty-nine hundred

1,250 = twelve hundred fifty

You can also say numbers like this:

I'll sell it to you for seven thousand five hundred dollars.

This is the more formal way to say it.