21 F70/100's for 15 million USD is about half what you would pay for a single new E190 (little bit less than half really).
Takes a lot of fuel to make up the difference. Bit of tarting up on the cabin side, slap in a new coat of paint and you have a cat 3 capable jet with about the same seat capacity as a 190.
Makes a compelling argument from a business perspective, particularly for low utilisation work where you might do a morning and evening service on a contract. No need to flog the machine all day, everyday just to make ends meet. With low utilisation the difference in fuel costs becomes less relevant, and the maintenance burden becomes less of an issue as there is more downtime for the engineers to do their thing to keep it going.