You can check 'completed auctions' on Ebay and CardTarget.com for prices.
Sovereigns aren't a rare or scarce back. On a scale of #1 being the most rare and #39 being the most common, Sov 350 Forest Green is ranked #31/39, Sov 350 Apple Green is ranked #22/39, Sov 150 is ranked #26/39 and Sov 460 is ranked #16/39. For more info on the backs, check out:
http://t206resource.com/Backs%20Gallery.html