The base price of a Sky was higher than the base price of a Solstice because of more standard equipment (mainly air conditioning and a lined top), as GS stated. Standard A/C is also why a Sky has been perceived to be heavier than a Sol, but two comparably equipped cars cost and weigh about the same.
Odds are the Sky will be worth less than the Sol because the perception is that a Saturn is less desirable than the same car as a Pontiac. That could be balanced (or even exceeded) by the fact that there are twice as many Sols as Skys.
A car's worth is determined by how much someone wants it or, in the case of an "under-valued" car, how desperate someone is to get rid of it.
If you could predict future values, or cars or anything else, you would become a very rich individual.