No idea, you really need to find a specific STM forum, where people use them - they aren't a very popular chip in hobbyist circles due to lack of support for them. The STM32 modules (which are incredibly cheap) connect to the Arduino IDE via a serial port, but I've no idea if they have a pre-installed boot loader or not?
Here's a site you might find helpful?:
**broken link removed**
But as far as cost goes, unless you're building huge number of a device, the small difference in prices of processors isn't really significant.