With Crimea effectively under his control, the question is - is Putin content or is he not satiated yet. No effective reaction from Western Countries, no conciliatory noises from western leaning Ukrainian Leaders in nodding to Russian interests (purported visit by former Ukrainian Leader Tymoshenko may be one way to sooth Putin) and open rebellion by Russian speaking Ukrainian population in southern and eastern Ukraine; all that will prompt Putin to be more adventures and invade Ukraine to bring all that Ukraine area as one more vassal state of Russia. His original intention, Plan A, was to get the entire Ukraine as a Russian vassal state, circa Soviet Union. Since his agent - uprooted former president Yanukovych - failed as well as Ukrainian population,especially western side folks, is in no mood to stay under Russian sphere of influence; Putin might think forceful separation of a large part of Ukraine is the second best option.
It is obvious that Putin has calculated that West would not be in any position to put troops in Ukraine and Ukraine itself being bankrupt; so now is the good time to go the Plan B. Trouble is outside of Crimea, any presence of Russian Troops would incite already arose population in Western parts of Ukraine and quite possibly we are likely to see an armed resistance by Ukrainian forces. Such a bloody and prolonged engagement would not be easy for Russia even though they would have an upper hand to start with.
If an armed conflict starts in mainland Ukraine, NATO would find it easier to beef up its help to Western inclined Ukrainian forces. Division of Ukraine will be a clear consequence then. That is where 'costs' for Putin and Russia would start biting - starting with getting kicked out of G8, sanctions would follow, Russian companies would lose in Europe and many other markets. Putin will threaten Germany by its supply of Natural Gas, but once outright war starts; Europeans will be more accommodating to other energy choices as well as politically it will be reasonable for European Leaders to rally the public away from Russian Gas. (Too bad that American Gas Bounty would not help Europeans.)
However, the biggest loss for Putin will be remaining parts of Ukraine - Western Ukraine - will firmly come into NATO sphere. Western countries would have seen Russia's return to Cold War geopolitics. Public in Europe and America will be more receptive to new NATO members and consequent obligations to defend those new members. (Otherwise also USA and UK seem to be obligated to defend Ukraine.) In effect, at that stage Putin would have achieved trade and economic isolation of Russia to the extent of Cold-War era as well as would have brought NATO further near to his doorstep.
For America and Europe to play this long game, they will have to show the nerve in backing Ukraine. Ukrainian leaders are asking UN to intervene here. UN will not be able to do anything as Russia is the veto member of UNSC. As Putin becomes more adventurous, Ukraine needs to ask for help from NATO. With a request to NATO, European and American interventions are possible. If there is no de-escalation by the time new government is in Ukraine, it can promptly apply for NATO membership.
Indeed this is a moment for President Barack Obama to show his mantle. He has been conciliatory for long. If America does not work through NATO to halt ambitious Putin, President Obama is unlikely to make any headway in Syrian conflict and Iran Nuke deal might not come as well. In all these negotiations and conflicts, Russia is the main backer of trouble makers. Emboldened Putin would practically make it impossible to get these conflicts resolved. America which has shown some 'spine and grit' in stopping Putin, would find it much more amenable geo-political situation from Afghanistan to Syria.
President Obama might not be interested in Cold War Chess; but that is what Putin wants and if left unchallenged, Barack Obama will be the leader who would have allowed post-Cold War Western consensus of global politics to go ashtray. Yes, we are talking here Munich 1939 in some respect, unless America acts. Luckily American Public will be more receptive for an assertive foreign policy with regards to Russia since more Americans know - effectively clapping Russian ambitions is a necessary condition to retain stable Europe. Americans would not like to go back to the 19th century politics where dictators decide how to carve out large swaths of world population. Time has come for President Obama to start putting in place a bit strong armed and deterrent foreign policy.