Us a postage or small kitchen scale (one with no more than a 32oz limit) scale.
First weigh an empty Pringles can with the lid on and record that number. Then weigh 10 quarters, divide that result by 2½ and you get the weight of one dollar in quarters.
Then weigh the can with the coins in it, subtract the weight of the empty can, and divide by the weight of one dollar.
Very simple (if you have the right scale and a small calculator).