What do you gain by having the shock resivoir? How much do they run $$?

 

Shocks with a reservoir are able to hold more fluid, and more importantly, keep that fluid cooler, longer. As the shock works(compressing over and over) the oil inside heats up and the performance of the shock decreases.
I don't know of any shocks that can be fitted with an after market reservoir. They come stock with an attached or remote reservoir. You may be able to replace a remote reservoir on shocks that were originally equipped with them, but adding one to a non-reservoir shock is not possible as far as I know.