When I buy a Mac, does it come with reinstallation media? Can I format my Mac? If I had one, would a full OS installation media be provided? Or at least an OEM version of it? If not, how much it costs to get one of these?
There's no such thing as an "OEM" version of OS X, since there are of course no OEMs for Macs other than Apple themselves.

All new Macs have recovery partitions on their hard drives that can be booted and used to reinstall OS X, so you can format your main partition and reinstall OS X. Failing that, they will also do a network boot over the Internet and reinstall OS X from Apple's servers. OS X also includes a program called Lion Recovery Disk Assistant that lets you create your own bootable recovery media on external drives, so you can make as many recovery disks as you want.

Failing all of these options, you can purchase Lion on a USB drive for US$69 (if you're in the US).