At SIGGRAPH, Richard Huddy of AMD announced the release windows of FreeSync, their adaptive refresh rate technology, to The Tech Report. Compatible monitors will begin sampling "as early as" September. Actual products are expected to ship to consumers in early 2015. Apparently, more than one display vendor is working on support, although names and vendor-specific release windows are unannounced.
As for cost of implementation, Richard Huddy believes that the added cost should be no more than $10-20 USD (to the manufacturer). Of course, the final price to end-users cannot be derived from this – that depends on how quickly the display vendor expects to sell product, profit margins, their willingness to push new technology, competition, and so forth.
If you want to take full advantage of FreeSync, you will need a compatible GPU (look for "gaming" support in AMD's official FreeSync compatibility list). All future AMD GPUs are expected to support the technology.