There is a models.js file at the following path in Point of Sale:

/point_of_sale/static/src/js/models.js

Look for module.NumpadState (probably line 1304). There is default values set here for the keypad, as well as in the reset() function, further down in the module. Changing the 'mode' in both these places seemed to work for me.

The defaults should now look like this:

defaults: {
            buffer: "0",
            mode: "price"
        },

and the reset() function (line: 1354) should look like this:

reset: function() {
            this.set({
                buffer: "0",
                mode: "price"
            });
        },

These changes should change the default selected mode to be 'price' instead of 'quantity'. I have tested this and it seems to work fine. Please let me know if this doesn't seem to be working correctly.