Just re-reading the Burwill offtake announcement again, they don't mention /t pricing, but state this:
"The exploration and testing data over the past 6 months demonstrated satisfactory results with spodumene-rich by-product concentrates up to 6.70% Li2O produced during commissioning of the original tantalum plant. "
So I'm assuming they are using 6.7% in their calculations (better to overstate so not to disappoint shareholders). From TAW's announcement the fixed price for 6.7% is US$985/t.
200,000t x $US985 = US$197M = ~US$200M
This aligns with the Burwill announcement:
"Starting from February 2018, BCL shall, in the first two years, purchase an aggregate of at least 200,000 dry metric tonnes ofhigh-grade lithium concentrate at a fixed price, worthing of approximately USD200,000,000. "