Surely it should be the fare basis. I.e. full Y to business, especially on the 767 routes where there is no Y+.
UA and US used to charge 10K miles for Y/B fares and 20k miles for lower fares. UA went up to 15/30 and US is going to 10/30 starting 1/1/03.
Actually, it is very generous of SK to allow upgrades from any economy fare class.