You are right, but they did not pay the premium on historical...

You are right, but they did not pay the premium on historical production of niobium vs phosphates, they paid a premium on the ramp up phase of the niobium business that will allow them to become the current second largest producer of niobium in the world:

"...The niobium business produces and exports ferroniobium and has recently completed a capacity expansion. It is expected to become the second-largest producer of niobium in the world upon completing its ongoing capacity ramp-up."

"The BVFR plant reached commercial production as of March 2016 and with the completion of the ramp-up it will support the niobium business to reach overall total annual capacity of 9,000 tonnes. The niobium business recorded sales volume of 4,600 tonnes and 5,100 tonnes in 2014 and 2015 respectively."

http://www.chinamoly.com/05news/DOC/E_03993_news_0429.pdf