In a general sense, I focus on paying to level skills that would take a long time to accrue manually:
- Whatever armor I don't use often (e.g. Heavy Armor for a sneak thief),
For the rest, I just alter my play style to have them level up quickly, or I use a number of exploits to level them quickly.
I leveled up block extremely quickly after level 53 by going into a dwemer ruin with a Master Dwemer sphere and just let it wail on my light armor and on my shield, and healing myself with a restoration spell. It took about 30 minutes, but I went from 50-100 in that time for block, and light armor went from 60-90.