Character Creation: Buying Attributes

Attribute dice pools are all bought with XP.
When first creating a character, there must be at least one die in each Attribute’s pool. Buying a new die in an Attribute pool costs XP equal to the current number of dice in the pool, plus one. For instance, buying the first die in a pool costs 1 XP, the second one costs 2 XP, and so on. When you buy a new die in an Attribute the die always starts as a d4. Each die can be upgraded to the next higher tier by paying the new tier’s level squared in XP. So, a d4 (a tier 1 die) can be upgraded to a d6 (a tier 2 die) by paying 4 XP. That d6 can then be upgraded to a d8 by paying 9 XP. To go up multiple tiers, you must pay for each step up individually. To go from a d6 to a d10, for instance, would cost 25 XP: 9 XP for the d8 and 16 XP for the d10.
Example: Frank's character is getting finished and currently has an Endurance of d8/d4. He has 23 XP left, and he wants to put it all into Endurance. First he bumps his d8 up to a d10, which costs 16 XP. He then buys another d4. Because this is the third die in the pool, this one costs 3 XP. His Endurance pool is now d10/d4/d4. He has 4 XP left, just enough to raise one of his d4s up to a d6. He has now spent all of his XP and is left with an Endurance pool of d10/d6/d4.