What is a Master Image?
The master image is the image used to generate your transformations (crops, resizes etc.). So you can request as many sizes you want and you will only pay for the master image itself.
Will I be charged by number of requests?
No. Requests are unlimited. Bandwidth too!. So it doesn't matter if you have 10 images with 1 million page views each. Price won't change.
I don't have 1000 images. Will I pay $1.00/month?
No. You will always be charged proportionally.
If I have 100k Master Images will I be charged by the same images every month?
No, only the images requested by your end-customer counts towards your usage. So what matters is how many images you used and not how many do you have in total.