Answer
About 25,000 dollars per month. In review, anything from 16,700 to 37,500 counts as right.
The worked estimate
500 TB = 500,000 GB. × $0.05 = $25,000 a month. Storing those bytes is often much cheaper than sending them, which is why a CDN's lower bandwidth price and caching matter for file and video services. (Prices vary by provider and volume.)
Numbers: Numbers to know.