Why does it cost 31 TDPs to raise my Gor'Tog stamina from 15 to 16?

The formual says it should be ( 3*15 ) + ( -2*15) = (45) + (-30) = 30 TDPs.

Belexor - Tog Barb
Eldaroth - Elf Range

He had the right idea, just didn't put in the whole equation.

( 3 * 15 ) + ( -2 * 15 / 2 )
so
(45) + ( -2 * 7 ) For some reason it does the current_stat/2 before doing the multiplication, and drops the fraction. It makes your favored stats slightly more expensive than you'd think, but your penalized ones a little cheaper.

(45) + (-14) = 31

There you go.

Killing you softly with his song,
- Stormsinger Shavay

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"Yer off the edge of the map, mate. Here there be monsters!"