You can keep the commuter parking lots and YRT/Go bus terminal at Finch, but build a small station at Steeles.
Even if you buy out the entire tiny block between Yonge, Steeles, Dumont, and Nipigon to build a TTC bus terminal, that's only 25 homes. TTC already has a small bus loop in it. At $500k a home, that's only $12.5m.
Some weekday parking could be acquired from Centrepoint Mall. The chunk closest to the proposed station is already used by a travelling carnival half the time.
If we build it today, I'd estimate the final cost to be at most $300m. The advantage is that we get really fast turnaround times, but we also reduce congestion on Yonge and pave the way for eventual extension to Langstaff.
BTW, speaking of Langstaff, the YRT terminal there and the GO station there desperately need a pedestrian walkway/tunnel across the tracks to connect to each other. The way things are right now, GO riders can't use a YRT terminal a stone's throw away.