I cannot see going under Highland Creek any harder than going under the Don River. When ECLRT was proposed to tunnel under the West Don and directly to Don Mills, the cost of that station was reported to be $60M.The Lawrence station has to be ten (yes, ten) stories underground due to the subsurface conditions.
All the TTC is willing to do (this stated at the meeting today) is design the route so there is a "level, straight" section where a station might be achievable some day. Roughing in the headwalls would represent 70% of the cost of the station, so that isn't an option.
Even the cost of fire exits (and ventilation shafts) must be horrific.
As for fire exits, it would either be part of the station. Actually, you could say 1 less fire exit would be needed because the station takes the place of 1 fire exit.