Users care about their money. Any mistake (for example, paying more than expected) will be very painful.
You should care about what your users care, so help them avoid mistakes.
When you put the price somewhere in the checkout form, users can easily ignore it, especially if they're in a rush.
If you put the sum on the checkout button, it serves as a kind of confirmation: the hope is, users will scan the label before clicking on it, so if it's not what they expect, they might not click it.
Of course, it's no guarantee, but it is a double confirmation tool for a cost of making the label a little longer.
It also helps ease anxiety for people not used to paying online - double confirmations help them make sure they are on the right track.