Very hard to say.
Lots of factors influence how much a Prius will cost in the future, including:
- Price of oil. Which is always changing.
- Time of year. End-of-model-year clearance sales affect price.
- Level of competition from other automakers (comparable offerings from other carmakers).
If current trends continue, you can expect the sticker price for the Prius to increase on average $300 or so per year.