Would it be prohibitively expensive to put crossing gates at every left turn lane?
The cost varies widely, the most obvious impact is the size of the intersection (length of any gates), whether you install dedicated rail warning signals (the norm), but also the style of gates and other factors can contribute.
The range is generally stated as $200,000-$1,000,000.
For a larger intersection like Eglinton, I wouldn't presume a penny under 500k per, and it could be the max.
So...what, something like 15 intersections in the east end? Up to 15M
I would not consider that prohibitive.
The barrier, pun intended, will be the impacts on signal timing, and transit priority.
If the gates don't come down ahead of the trains and hold until they clear the intersections, it will cause substantial delays to the TTC; but if they do, it will mean giving Eglinton more green time at the expense of cross streets.
Where trains (going opposite ways) don't cross the intersection at the same time, (which would be most of the time), the time impacts on traffic could be substantial.