In fact, you may be losing some features going from 11 to 13. And, you will not be able to use 13 on Windows XP, Windows Vista, or Mac OSX 10.7. Please refer to
We are not Adobe. Rather user to user. If you purchase the product as a download from Adobe, you will find "upgrading" price. Adobe means if you have an earlier version of the product, you can get it at a lesser price that if you did not. If you buy elsewhere, you are governed by the pricing policy that the seller uses.
Shop around online. Remember that Elements programs are standalone programs and "upgrading" is seen only in Adobe pricing.
The following links may give you better insights
Please review the above. Any questions or need clarification on any of the above, please let us know.