Don't forget about the gas pipeline in play. The Ukrainian economy is a complete mess and it has long been "borrowing" Russian energy supplies pumped through its territory and destined for Europe. Only recently it threatened to be even more energetic with its pilfering. This instability is one reason why Russia and Germany developed a project to deliver energy via Northern Europe, a project opposed by Washington where the goal is to cripple the Russian economy by disrupting one of its major exports. No thought about it fucking us over here in Europe either. That's what allies are for.
The cost to Russia is access for its fleets via the Black Sea and its energy export markets. Major wars have definitely started for just these types of provocations. The warmongers in Washington set their own project in motion with the overthrow of the Ukrainian government in 2014. It is well worth looking into who they chose to replace the legitimate government with. Take a quick glance and it all starts to smell of Syria and Afghanistan again - they only call them terrorists when they are off the payroll.