With numbers like that, the batteries don't sound all that expensive. How many batteries you would need per bus depends on a number of factors. Charge time is a big one.
Well, a quick search shows 260-360 horsepower for buses. An 85 kwh Model S is 362 hp. Now, I know that HP is far from the only factor, torque is as well, which is why buses and other large industrial vehicles tend towards massive diesel engines rather than fairly small gasoline engines that produce more 'power' on paper by the horsepower spec.
Mainly because the smaller engine will tear itself to pieces in short order if asked to do the duty cycle of the bigger engine. Still, electric motors are notoriously tough, but to my thinking a model S drivetrain would be an excellent stand-in as being for hybrid components for a bus.
Now, a Model S manages 265 miles off it's battery with a vehicle that's extremely aerodynamic and only weighing 4,647.3 lbs*.
Meanwhile a bus isn't aerodynamic, and I'm seeing 22, 28, and even 40k pounds.
That gives me electric ranges of 56, 44, and 31 miles of range per 85kwh battery pack, if you figure that lower average speeds allows the battery pack to mostly scale linearly. I'm also seeing 4 mpg for a bus - which would translate to 24 mpg for the model S of you compare the 28k pound 44 mile and multiply by the divider, so it sounds about right.
In addition we know that the wheelbase on a Model S is 116" and that the battery fits between the wheels. So a 45' bus should be able to fit at least 4 of them, assuming that with a width of up to 102" you couldn't fit the batteries in sideways.
That's without figuring on stuff like stacking the batteries on top of each other. My conclusion is that there isn't any need for roof mounting, removable seats, or even trailers for extra battery storage. If you want to save the battery weight, simply unmount them and leave the packs back at the station.
*I'll note that for a car the model S is actually pretty heavy, but we're comparing it to a bus here.