"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.