Strictly speaking, he hasn't 'invaded' Donetsk and Luhansk, they were already under control of Russian separatists since 2014 and were recognised as an independent State by Russia. It's basically the same as England sending troops to Northern Ireland for peacekeeping duties It's not cheap, but this is what armies are for, as an alternative diplomatic tool. America didn't like Cuba hosting missiles and Russian doesn't like a furture Ukraine being used to host missiles either. Remember Trump sent the Ukraine Javelin anti-tank weapons, but they are useless against non-armoured separatists, they are only useful against Russian tanks.