The use of cheap drones by Ukraine to attack Russian war jet fleets might be the reason why OP assumes that war is cheap. But it's great that many responses have made it clear that war is the most expensive project a country can undertake. The amount of money that has been spent in Ukraine and Russia could be enough to turn a third world nation to a developed one.
I would have thought the OP was taking about how it's now too easy to start wars because I still don't get why Russia and Ukraine are still fighting. Aren't they seeing the amount of people that they're killing and the turning of their life upside down of those barely surviving. War has always been expensive because of the weapons that has to be purchased by the country that has to win. What wins wars isn't the human power that you posses but the weapons you have access to and this depends on the amount of revenue that the country has. There are country's that you can't have more weapons against hence having war against that country is a waste of your human resources.