EV mode

No, that is preset by Toyota based on the HV battery size and the desire to protect the battery. From the Owner's Manual:

Possible driving distance when driving in EV-Drive Mode is up to about 1/2
mile (1 km) if driven at a speed of about 25 mph (40 km/h) or less. (Possible
driving distance depends on the hybrid battery [traction battery] level and
driving conditions.)

 

(And remember, you do not want to be in EV mode anyway, unless you are going a VERY short distance, like across a parking lot)

 

EV mode will cost you more money if you use it to "drive" rather than just move the car to a different location in the parking lot or your property. The reason is the power from the battery comes from gasoline, generated and used at about 50% efficiency compared to just using the power from the gasoline to propel the vehicle.
That being said, you can save -some- money/fuel by using EV mode to avoid having to warm up the car if you're just moving it a short distance. The warmup cycle wastes a lot of energy. It's the main reason short trips kill your mileage.

Lastly, using EV a lot will very likely shorten the life of the hybrid battery. How much is debatable. But if you "use it up" you're looking at a $4000 hit to replace it.

Now, some of what I just posted above will be negated by a Plug-In Prius. However, the energy it uses in EV mode comes from the electric grid, and you'd have to do the calculations to see if it is less expensive than energy you derive from burning gasoline. This will vary from place to place, due to electricity costs varying and gasoline prices varying.