Penny Floors are not illegal.
As long as you've acquired the pennies legally then you can legally use the pennies to make a penny floor or use for any other legal purpose as a building material.
If you're looking for durable flooring it doesn't get more durable than this.
By the time you've glued down and grouted your pennies you've already got a tough durable floor that will stand up well to high traffic.
Depending on how you lay your floor, you can expect to spend about $3.00 per square foot in pennies.
Don't have that much spare change lying around? Your bank will likely let you buy from them.
Lastly, you'll need to keep longevity in mind.
256 pennies per square foot if the rows are straight.
No penny overlaps any edge and there's no space left whatsoever - at least that's what math tells us.
Because of the pennies' shape and placement on the penny sheets, the smaller gap where the pennies are closer to each other is about 1/16th of an inch BUT the rest of the gaps between pennies are bigger, therefore the need for sanded grout.