How much drag should I expect with the rear wheels with parking brakes off?

Good catch, Jonny....it takes very little drag to reduce mpg. By design design disc brakes do have a LITTLE drag, drum should have none.
I drive an antique Prius with drums rear, sorry I don't know which you have on back on the 2012. But if one side can spin drag free it's a given that something's fishy in Denmark.
If you have combined disc and drum on the rears you can pull the caliper to isolate problem to disc or drum area.
Causes of sticky rears are....
Sticky or tightly adjusted cable...grab cable and wrestle with it behind the backing plate, a sticky cable will often let go with massage (replace cable).....
Drum brakes ... star wheel too tight, or defective/unhooked spring , or unbonded shoe lining, binding in wheel cylinder or restricted flex hose


Disc brake.....slightly worn piston cocked in bore or hanging up on hard o rings, dry or defective slider pins, restricted flex hose.

All the above can be checked by a good mechanic in about 20 minutes, or by a Toyota service advisor in half a day.

 

I checked our brakes, and re-assembly of the rear was a bit of a learning curve. There's a parking brake mechanism incorporated into the rear caliper pistons. They have a rotating screw with a spring wrapped around it. The spring allows the screw to unscrew only, slightly extending the piston length, to reduce pad to disk clearance.

The rear caliper's piston has a raised x pattern on it, that has to be in the correct orientation at re-assembly, so that the cross straddles a pin on the back of the inside pad. This prevents the piston from rotating. To get the orientation correct you may need to screw the caliper clockwise slightly, created some play in the process. Anyway:

I'd done the brake work with battery disconnected, and when done, before reconnection of the battery, I made a point to depress the brake pedal multiple times, with the aim to pressurize the system, and avoid the car's electronics noticing anything, like excess pedal travel, etc.

It worked fine, but one thing I noticed: with maybe the first 4 or 5 pedal depressions, I heard a fairly loud mechanical clicking. The next few pedal depression, no sound.

I believe that was the rotating spring-wrapped screw, taking up the slack, reducing the gap. In other words it's auto-adjusting.

After that brake job I spun the rear wheels and found them a little reluctant, there definitely was a bit of drag, but not signif. I gave it time, and one thing I would do is feel the wheels after an extended drive. Go around and feel all four wheels, to see if one (especially a rear) felt warmer. At present they all feel barely warm to the touch, none significantly different than the others.

My take is that the rear auto-adjusting mechanism is pretty bullet proof, takes up slack, may be slightly snug immediately after re-assembly, but works itself out with a bit of pad wear.