Some possible reasons:
- One of their sources is Sudanese gold. Basically, they're helping to exacerbate the genocide in Sudan, so they can fund their genocide in Ukraine.
- Russia has arguably won the sanctions war. They've shifted trade to China and India. Or they're exporting raw materials through third countries to Europe, or transfering oil at sea, bypassing sanctions at a relatively low cost.
- Made deals with other oil producing countries to not increase production, so that prices remain stable.
- War isn't that expensive, unless you're the US (and care about casualty rates). North Korea. Dirt poor country, almost entirely cut off from the world, still manages to maintain a sizeable military and even develop nuclear weapons. Russia? Old soviet stock that was rusting in a barn, North Korean shells that occasionally kill your own troops, troops you only pay if they survive if at all. Send prisoners to the front, saves money on prisons. Send people from the poorest regions, so you don't have to pay them much, and it doesn't negatively affect GDP that much.
- Do like the Russians did during and after WW2. Rape and pillage. Russia is actively stealing Ukrainian resources, equipment and grain from the occupied territories.
- Sell (or barter with) pariah states like North Korea or Iran military technology they would otherwise not have access too, like nuclear or ballistic tech.
- War time economy means you can cut lots of stuff. Education, pensions, healthcare, drugs... all very expensive. Not as if the kids dying at the front will need an education anyway. Hell, apparently they're sending disabled soldiers back to the front. Great way to get healthcare spending down.