According to some of the Ukrainians, Kherson is a key to Crimea over the long haul (unless Putin builds a bridge across to Kerch), and Kherson also lies on the path (more or less) to Odesa. Kherson is not very Russian (25%, I believe I saw on one map), so that's where things could get sticky.
http://www.angelfire.com/extreme4/kiddofspeed/maydan/map.gif
"Kherson is a key to Crimea over the long haul"
If Russia goes to Kherson they're not going to stop until they're through Odessa and into Transwhatever.
The costs to them would be the same either way.