Here is a very nice report from Fraser Mackenzie on potash in general and reports/comparisons of the following potash companies: AAA, ELM, FED, ICP, KRN, POT, STB, NPK and WPX.
South Boulder is on page 70. Fraser Mackenzie has an opex of $110/ton. Dundee Market Analysts have an opex of $105/ton for STB.
It takes a while for the file to load:
http://www.frasermackenzie.com/newresearch/POTASH_COMPENDIUM/FINAL_RPT_POTASH_Prattas_191211.pdf
GLTA,
Sam