Some thoughts on requirements for taking spheres.

Each discipline has 2-3 "favored" spheres (see ExaltedHackingSphereDistibution).  This does not affect the cost of the sphere in terms of XP, but does affect the requirements to take it.

Most spheres have 3 levels.  The first level costs 2 x as much as an Occult charm, the second 3 x as much, and the third 4 x as much.

Some spheres instead have several parts (like SphereElements with air, fire, water, earth, and wood).  These are assumed to cost as much as first elvel spheres unless othewise specified.

The minimum essence required to take a sphere is based on the level.  For favored spheres, the first level is 2, the second 3, and the third 4.  For unfavored spheres, the first level is 3, the second is 5, and the third would be 7.

For collections, the first sphere bought has a level 1 requirement, the 2nd and 3rd spheres and level 2 requirement, and the 4th and 5th a level 3 requirement.  So an elementalists can pick up one element at essence 2 for 16 xp, two more at essence 3 for 16 xp each, and the last two at essence 5 for 16 xp each.  A smith, on the other hand, could pick up one element at essence 3, and one more for each point of essence above that--he'd need an essence of 7 to get all 5 spheres.

There are also other reuirements to take a sphere, which vary by sphere, and there might even be several different ways to approach it (different skills, stats, or even virtues).  An occult of at least one is usually required, though.

odd idea--Karma Ritual requirement to take a sphere? (as attachment to your discipline)