Ukraine

Putin is afraid to force people into service too often. When he mobilized a lot of Russian men left the country, to avoid the draft. So he instead pays a massive sign up bonus to get men into combat. However the economy has worker shortages already, unemployment is nearly gone, massive emigration due to migrants and young Russians trying to avoid the draft, lack of new migrants coming to Russia and low birth rates for decades mean that wages go up everywhere. So sign up bonuses are high. That is two years of the average Russian salary already.

So in a way he is forced to pay. The alternative might hurt the economy even worse.

Mandatory military service in Russia lasts only 12 months and is required for men aged 18 to 30. To maintain troop levels, Russia has to rely on paid contract soldiers in addition to conscripts.