With the cost to date of $9.3 billion for Line 5, and $2.9 billion for line 6, the cost is about a third of that of subway, not half (yeah, even when you correct for the length of surface vs underground and the line lengths - two equations two unknowns).
And that's just the construction. The long-term O&M (not including TTC operations cost) is closer to one-quarter.
Meanwhile the 18-km Mississauga transitway cost $600 million a decade ago. And about a decade ago the 19-km Waterloo ION LRT cost $820 million.
Perhaps it's not as simple as BRT is 1/10th the cost of an LRT. Perhaps you mean applying red paint on roadways?