...How much do 75,000 bottle caps weigh? It would be fun if there was actually weight to currency. It would force using safes at houses. Plus if you knew you were on your way to make a large purchase at the Gun Runners, you would have to drop some gear to temporarily transport the caps. Kinda cool huh? Any mods out there?

You could modify the weight yourself.

There have been mods to add weight to coins for the purposes of realism in many moddable games, so it wouldn't surprise me if someone had done this already or was in the process of doing it, try asking over at the New Vegas mods forum Or open up the GECK and do it yourself

but what if he is on console?

I think there's a mod or two on Nexus that adds weight to bottle caps. As for 75,000 of them- If you figure each cap weighs about .005 pounds (say 2.5 grams each), then that'd be about 375 pounds of caps. Definitely time for a safe, locker, footlocker, bank vault, something.