Russia has started to fly Shahed drones much higher after Ukrainians became effective with shooting low-flying ones with mobile low cost AAA guns. This made drones easier to detect with radars and shoot with missiles, but missiles cost like 10-100 times more then drones and is not sustainable.

Russia also started to deploy mobile anti-drone guns and there a lot of vides that show their effectiveness but Ukraine still fly drones low as Russia still willing to use expensive missiles against them on massive scale.

The issue is not the cost, but availability of AA missiles. Russia is capable of sending 500+ drones in a given day. After few weeks/months any stockpile of missiles will be consumed.