wll thats the cuestion, i am doing it myself with domains at war battles conversion, and i have somthing in the lines of incresing the wages of each elven mercenary by 15gp.
If they're all level 1 spellswords, it should be the same cost as elven whatevers plus the spellcasting bonus to BR from D@W:B pg 94.
So, 500 * 1 * 120 /24000 ~= 2.5 if it's an infantry unit.
Generically, the +2.5BR for being an L1 spellsword (multiplied by 120, div. by 720) comes out to 15 GP, so, yep.
You might find that applying the BR bonus to an actual unit gets you a every-so-slightly different number.
Elven Light Infantry is BR 1.5, add 2.5 to get BR 4 for the unit.
Back out to the individual level, that's 0.033 per elf, multiplied by 720 gets us (23.76~=) 24 GP. Regular Light Elven Infantry makes 10gp, so thats +14gp for spellswords.
Elf Heavy Infantry A = BR 4 += Spellcasting BR = 6.5 =~ 39GP, or +15 GP
Elf Longbowmen = BR 7 += Spellcasting BR = 9.5 =~ 57 GP, or +15 GP
Cavalry would go as 500 * 1 * 60 /24000 = BR 1.25.
Elf Cataphract Cavalry = BR 13.5 += Spellcasting BR = 14.75 = 177 GP, or +27 GP.
presuming I did that right.