Are you seriously saying it isn't worth $50?
http://bohemiancoding.com/sketch/support/faq/01-purchasing-s...
Worth noting I am not a professional designer. This is a hobby purchase - so yes, being hit with an extra $50 expense a month and a half after deciding to splurge and buy the thing feels harsh.
Also, what relevance is the 20 days figure? You owned the product for 63 days before the upgrade was released.
The cutoff date for getting version 3 for free is 1st March.
How far away from the cutoff you were is irrelevant. What is relevant is how soon after your purchase the upgrade was made available.