It's a refund on the interchange fee from not doing chargebacks and paying your bills on time. Your data is not interesting or valuable, that's your ego talking.
Data brokering is a 200 billion dollar industry. Some individual data brokers say they have data on half of all transactions in the US, so it's safe to say that your transactions are more likely to be sold than not.
Nobody cares about your data (unless you're high profile), but -- for example -- hedge funds and the like will spend big bucks to get aggregate sales data before this quarter's financial reports are written.