Interesting... Edge of the Empire (Star Wars RPG) has similarly costing skills and talents, but there is an incentive to upgrade talents, since the talent trees are the only route to upgrading your base attributes. I like this a lot. Skills can always be improved, but there is always a huge draw to invest in talents.
My Year Zero hack will be Career-based (think WFRP), so I can basically dictate the maximum number of skill increases allowed per career. Then I will make every upgrade cost the same. I am also trying out various talent tree implementations to tempt players to go for talents rather than skills.
Not that this helps with RAW in Forbidden Lands, but houseruling shouldn't be too hard if you're not happy - for example, make everything cost 5XP and don't allow skills to be upgraded too often, saying that your players need to choose at least one talent in between each skill upgrade.