N does not count since it takes more N to breakdown the straw than the straw contains so removing the straw makes more N available for the next crop.

Last bean straw bales I tested were .14%P and .88%K based on DM which works out to 6.41lb P205 and 21.21lb K20 per ton on 
20% moisture bales.  Given .24/lb Urea, 0.37/lb DAP, and 0.23/lb Potash it works out to $6.71 in nutrients for a 1400 5x6 bean straw bale.