For real-time streaming API a low cost option is IQFeed.net which charges about $67/month for Nasdaq, AMEX, and NYSE + $50 setup. The data feed source is ComSto

For real-time streaming API a low cost option is IQFeed.net which charges about $67/month for Nasdaq, AMEX, and NYSE + $50 setup. The data feed source is ComStock and the only API available is through Windows COM (yeah, I know) To offer real-time to the public you will need to set up agreements with each of the exchanges individually. Expect to pay about $1/user/month plus some fixed monthly fees. If real-time isn't essential, you could also use the fantastic python library ystockquote to get delayed data from yahoo.