cars

Your second cheapest option (aside from just keeping your eye on the market for a used one) is to put one together from a Royal Enfield and a Cozy (or other Indian/Chinese sidecar manufacturer), but you're still getting up to at least $15k (maybe as low as $10k if the RE is used when you buy it. maybe as low as $7k if you can marry both a used bike and a used sidecar) especially if you get a shop to actually mount it for you (which is highly recommended since sidecars can be a bit squirrely even at the best of times). This would be the easiest to register too since it's a modern international motorcycle company.

The cheapest option, if you're willing to go through the effort required, is to buy a refurbished Chiang Jiang from China. They made a million military motorcycles in the 70s and there are a few sites that purport to be able to sell and ship one stateside for about $5k, but idk how tarriffs and stuff play into that the price has probably gone up since I last researched it. These can probably be registered as vintage vehicles wherever you live.

Should be noted that with these first two options you're not going to be driving on the highway with the bike in question. The 70s military CJs don't have the power, and cheap sidecars aren't safe at highway speeds. There's a reason new Urals start at $20k - they've got the suspension/brakes/etc to handle going over 100 kmh.

I also see a listing on Alibaba for a modern CJ with an 800cc V twin, which might be from the new company called CJ in Taiwan that released their modernized CJs a couple years ago. If that listing is legit then that may be the best option overall, although idk if it's registerable in whatever country you're in and idk about its performance/safety.