Ok, let's run through the basics. You start at Legend 3, which means you can have up to Epic 2 in any particular ability. You can, if you want, put all 10 points into knacks, and nothing into boons from your starting points, so there's 10 dots worth of epic abilities as well. You also have your 15 bonus points. Another point of Epic is 5 of those 15, or 4 if it's favoured. So, if you buy non favoured epics with the 10 dots, and favoured with the bonus points, you can potentially save yourself 3 bonus points to spend on other stuff.
Before you spend any xp, there's 13 points worth of epics (max Epic 2)
Then you have 300 xp to spend on Epics, capping at Epic 3.
Buying the first dot of Epic (favoured) is 8xp
Buying the first dot of Epic (non favoured) is 10xp
Increasing a favoured Epic from 2 to 3 is 8xp
Increasing a non favoured Epic from 1 to 2 is 5xp
Increasing a non favoured Epic from 2 to 3 is 10xp
So, if you buy 1 dot of Epic everything from your starting points, you save on xp, which can be used to increase abilities and skills, or buy boons as well.
Next we go to Legend 5, and add the Demigod powerups. At this point you're capped at Epic 4, so you may have less options on where to put those points. It still probably works out cheaper to boost your non favoured Epics from here. After you spend those points, you have the bonus points again, which could get you another 3 epics.
Now you move automatically to Legend 8, and have 300 more xp to spend.
Let's assume you're on Epic 4 at this point.
Increasing a favoured Epic from 4 to 5 is 16xp
Increasing a favoured Epic from 5 to 6 is 20xp
Increasing a favoured Epic from 6 to 7 is 24xp
Increasing a non favoured Epic from 3 to 4 is 15xp
Increasing a non favoured Epic from 4 to 5 is 20xp
Increasing a non favoured Epic from 5 to 6 is 25xp
Increasing a non favoured Epic from 6 to 7 is 30xp
It's expensive, yes, but certainly doable for at least the favoured epics. Keep in mind you have to raise your attributes too, which effectively runs up the cost of your Epics.
Does that help any?