One power cut and your cryptos and nostr is dead on arrival.

How much it costs to setup a Msshtastic?

Ever heard of Cassacious Coins? You can trade those when there is no power. Just make something similar for Monero and you'd have no concerns.

Never heard of it. How does it work

They were physical bitcoins worth 100 Bitcoin, 50 Bitcoin, 25 Bitcoin, 10 Bitcoin, 5 Bitcoin, 1 Bitcoin, etc. A couple of years ago. But the state did not like them, so they made them stop. They had the private keys under like scratch-off holograms, so you could tell if it had been tampered with, in which case it would not be acceptable. But as long as that private key was intact, you knew for a fact that it had the amount of Bitcoin on it that it said it did. Since they were physical and had the private keys associated with the physical coin object themselves, it was totally off-chain and therefore needed no electricity to trade.

Pretty cheap. But with tariffs what was once cheap is probably more expensive. Cost depends on what you want to build

Interesting use of physical coins. Do you think they would have sold at a premium to Bitcoin's price?

Can do this with opendimes now, not as fun but just as bearable.

I never had any myself, but I do believe they sold at a slight premium because of the work that it took to make them.

Define cheap

base cost same as a raspberryPi pico with a few extras. But you can really go wild. Go look at their devices page, and see what you want to build then start there. If you don't know, then do another project first before tackling this