Denomination has nothing to do with how scarce one or the other is. It has to do with production numbers. http://www.mycurrencycollection.com/reference/star-notes/lookup/1/2013/F00498366
If you have a $20.00 star note would you share the serial number with us? Maybe we can help you get some good information on it.
You can look it up on the star note lookup website. The date is also useful, and of course condition. If it's all creased and stained it's $20. Almost every $20 star note to me, is $20, but some have lower print runs, and if it is super crisp and clean and brand new looking, it can be worth a little more.