Quality / moment being equal:
Cover > splash > variant cover > regional variant cover, IMO.
What's happening, who's present, and visuals are very important [to me]. Look at the DPS of the Supes/Batman fight from Hush VS the 617 cover. Went for the exact same price, down to the dollar. That said, the BvS image was a whole lot better IMO.
I think it's too difficult to affix a formula that would generally apply. There's too many exceptions, and some comics where covers and splashes are equal.
Look at Long Halloween, splashes are way better/typically go for more than the covers due to the simplicity/object nature of the covers.