"Price per 1000 units. Unit volumes are based on monthly usage."
It's $2.50 per 1000 units, so 0.25 cents per unit.
Edit: And, according to the pricing page  the first 1000 units are free.
For higher volumes, there is also the OCR.space api. It offers 25,000 free conversions per month. It is not as good as Google, but works fine on screenshots.