The dollar bills have a slight hollow indent, so you can’t just model them as a solid prism of ABS. I assume is the question here. You might be off by about 15%
The part pictured here seems to be 3069px7 with the base color incorrectly set to white. In any case, it’s 3069, the standard 1x2 tile. Thanks to the folks at LDraw who have modeled every Lego brick in detail (because of course people have done that), we get a volume of 303.8mm³, with a bounding box size of 409.6mm³, for a density of about 74%. But, Bricklink can just directly tell us the mass of a 1x2 tile is 0.26g, so the total mass is 10.5 metric tons.
Of course someone has just published a table with all know masses of all lego pieces ever
Bricklink is a site for individuals/small business to buy and sell primarily individual Lego pieces, so it’s important for shipping calculations to have reasonably accurate weights of all the pieces. Their weights are therefore contributed by those sellers. Although now that LEGO Group owns Bricklink, you’d think they could just slide them the numbers.
I used the weight value on bricklink.com which I assume is correct :)
