How much does it cost Russia to send warplanes close to NATO airspace and then back to base?
Are other forms of wartime posturing, like submarine patrols in enemy waters also nontrivial costs?
Compared to the total cost of their military spending, not very much, but it certainly costs them something.
On the other hand, they want their pilots and submarine captains to be constantly ready for war so what better way to do that than through posturing maneuvers.
Except another word for this sort of posturing is “training”. If your job as a fighter pilot during a war is to intercept enemy aircraft and then take action, a good way to practice doing that is to actually do it.
This was a common practice during the cold war for both sides.
You’ll save a ton of money if you don’t fly missions with your planes and leave your subs docked, but then if/when the time comes when you actually need to take these weapons into combat, their crews will be largely untrained and inexperienced. Aside from political posturing, these missions also train their crews for readiness, and also train their crews to deal with real world stress, which is something that a training mission over friendly territory wouldn’t accomplish.
It also gives you a chance to see what weapon systems your potential enemy is also training with and what new types of radars, sonars, etc. they are using to look at you.
Not Russian planes but here’s the cost-per-flight-hour for various USAF planes and you can assume the Russians are paying similar amounts.
When the Russians fly up to the edge of US airspace, they also listen to see what sort of radar detects them and when, they probably listen to radio chatter to see what happens in terms of scrambling intercepts, where they come from, response times, etc. It’s more than just posturing, it’s also keeping intelligence up to date about US capabilities.
(The US did the same to Russia. One theory is that the KAL007 (747) was shot down because it was confused with and mistaken for an intelligence radio surveillance plane (707) that regularly trolled the Siberian coastline. The Russians thought “aha, got you now” when the aircraft actually overflew Kamchatka, and shot it down.)