On p.48 of the Rulebook, under Non-Career Advances, it says (emphasis mine):
Characters will inevitably want to advance Characteristics and Skills not listed in their Careers. That’s fine, if the GM feels it’s appropriate, but it costs double. The GM might want you to find a teacher or otherwise play out this unusual education.
Non-Career Characteristic and Skill Advances cost double the amount of XP listed in the Characteristic and Skill Improvement XP Costs table.
So this is fine at GM's discretion and costs double the usual XP. But then under Endeavours there's a different set of rules (the Training Endeavour, p.199), which says:
The Training Endeavour allows you to train in a Skill or Characteristic outside of your Career, treating it as if it were in- Career. In addition to the normal XP cost, you must also pay money to the tutor. Training for Basic Skills and all Characteristics costs XP+1d10 brass pennies , where XP is the number of XP it costs to buy the Advance. Tutoring in all Advanced Skills costs double that amount. For more, see Advances, see page 43.
So, which is it? Is it GM's choice which you need to do? Or are these separate rules offering player characters two options for Non-Career Advances, one simple option at double the XP cost, one at the normal XP cost + money + an Endeavour?