It’s nowhere near as expensive as it sounds, and much more humane: offer every Russian soldier who defects $100,000 and the right to settle in any EU/North American country of their choice. It might not work, especially if Russia uses deadly force against soldiers who lay down their arms or violent reprisals against their families back home. But if a large enough portion of their troops accept the offer, the Russian war effort would be crippled.
Of course, Putin will claim that this is an inducement to the worst sort of betrayal, selling out your country’s fundamental values and interests in exchange for a bribe. If this were really what’s at stake in the war, he’d be right. But if not just a few, but thousands, including whole units, opt for the deal, it sends the message that Putin’s war aims are hollow. Of course, defectors can send that message directly as well, for instance through social media, and it would take a much higher level of repression than today’s to block it.
Too expensive? Way cheaper than war in every way. As an upper bound, consider that if 100,000 troops defect—which would likely end the war on the spot—the cost would come to $10 billion, far less than what will be spent on arms and lost through death and destruction.
Note that this is a weapon only the West can wield, since Russia can neither afford such largesse, nor would settlement in Russia be as attractive.
The one big disadvantage with this approach is its intrinsic injustice: Russian soldiers get this deal, but not Ukrainian ones. Giving the same bounty to Ukraine’s troops would be much costlier in relation to the value of outcomes achieved, and there’s the additional problem of extrinsic incentives crowding out intrinsic ones: soldiers who fight for cash are less likely to display the same courage or take the same initiative as those who fight for a cause, or simply for each other. (The dreadful record of mercenaries hired to fight the US war in Iraq makes this clear.) As a substitute, I recommend that any announcement of bounties to Russian soldiers be accompanied by a credible pledge of even greater funds to rebuild Ukraine after the war.
Bribery is an unsavory basis for public policy, but murder and mayhem on an industrial scale is far worse.