If your flight you booked is full then of course it's natural that the upgrade would cost more. If you are flexible in when to travel, you might have a look at the load in Plus of Business and find a flight that has a light load on those classes.
I don't think SK Y seats are getting worse by the year - Y is cramped if you compared to Plus/Business and have you tried the new UA 777 in Y, that's cramp even in E+.