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?
Costs have been mentioned. I feel called upon to be annoying.
In terms of raw cost, the cited numbers for 5 and 6 is 9300m and 2900m, however accounting for inflation I put these at 11200m and 3100m respectively.
Thus, per km, we get ~600m/km for 5 and ~300m/km for 6. Assuming the 7 km of on-street section of 5 was built at the cost of 6, the on-street section cost 2100m to construct, and the underground 9100m, for an underground cost of 750m/km, or 2.5x 6FW.
I would also caution "Well, this shows that on-street LRT is definitely 2.5x cheaper than subway," because the 6FW was drastically more expensive than it should be. 10 Hurontario, ION Cambridge, and Hamilton LRT are all set to be cheaper than 6fw (Fingers crossed!), and 5 also had its finaegering wrt to costs.
The Mississauga Transitway and ION are a bit more interesting, and are hard to compare for costs due to reused ROWs which is rare in Toronto. MT excluding the highway/reused is 10.3km total for ~65m/km, and ION including the reused rail ROW is 58m/km. Again I would not read into these numbers too much because they are whole different beasts- the MT is a whole new ROW while the ION reuses existing streets and old rail ROW.
A bit more fair of a comparison would be Viva, which was 67m/km of dedicated BRT, and the planned ION Cambridge which is estimated to be ~200m/km.
A final TL;DR on my thoughts wrt to prices for solutions, in the current context of Toronto: Median BRT is ~70m/km, LRT is ~200-300m/km, and I can't say I have any good data on subways/grade separated- but I think 700m/km is a reasonable lower bound, and the recent subway projects in Toronto are set to be far higher to the potential bn/kms.