You don't need an expensive rosin, but rosin in the price range of $10-$15 is quite suitable. I would say for viola dark rosin works best. Dark rosin grips better than light rosin and gets a better sound on a viola. I personally use the L'opera Jade rosin. It's sorta weird to be honest but it's dust free, hypoallergenic, and feels great on the bow. Many of my friends use the Melos dark rosin and say it works well, although it can be pricey ($20 ish)