As we've all just witnessed the majority of footballers are in it for the money, so I'd guess that the win bonuses for the cups are far higher than for the SPL. Its the only way of explaining the difference in performances against Copenhagen, Moscow, Dnipro versus St Mirren, Gretna & Falkirk.
The UEFA bonuses agreed at the start of the season were likely to increase at a stupid rate depending on progress as I doubt anyone thought we'd get past the league stages.