There's a slight difference in cost in the alternatives between "boots on the ground" and "doing nothing", you know. See: Libya.
Neither Iraq or Afghanistan cost 4-6 trillion. But even if they did, "war" with Syria would almost certainly involve NATO enforcing a no-fly zone and maybe blowing up some things from the air, rather than a full-scale ground invasion. But even beyond that, if we needed to raise 4-6 trillion for some reason (say, over a decade), I have no doubt we could do it fairly easily. Our interest burden, I believe, is lower than it was in the 1980's as a percentage of our economy.
TIME TO BRING SOME MOTHER ****ING LIBERTY TO SYRIA!!! <object width="560" height="315"><param name="movie" value="http://www.youtube.com/v/7R5A0pg4oN8?version=3&hl=en_US"></param><param name="allowFullScreen" value="true"></param><param name="allowscriptaccess" value="always"></param><embed src="http://www.youtube.com/v/7R5A0pg4oN8?version=3&hl=en_US" type="application/x-shockwave-flash" width="560" height="315" allowscriptaccess="always" allowfullscreen="true"></embed></object>
Even though he started the thread and has posted several times, bass-troll is too much of a coward to say. He just wants to wait on Obama's reaction and criticize whatever he does, no matter what.
Even just looking at combatants though. It's not like this is all Islamist militants vs Syrian army thugs. There are lots of good people fighting for freedom from an oppressive dictator there too.
http://www.huffingtonpost.com/2013/03/18/iraq-war-costs_n_2885071.html This estimates 1.7 to 6 trillion in total future costs
As far as I can tell, your chart pulls "6 trillion" completely out of its ass and even manages to get around it by claiming "over the next few decades", which thus means that by the time its claims would be proven wrong, no one will remember it, and also accounts for things which we would be likely paying for to some degree even if we had never invaded. And that's not even getting into the fact that no one has actually advocated directly invading Syria. There's plenty of good arguments against helping the rebels, such as the fact that they're no better than Assad human-rights wise and are too decentralized, without going into apocalyptic rhetoric about how the US will be destroyed by such an invasion.
That sounds like a useful, well-developed range - only $4 trillion in variation over "decades". Might as well say we estimate total future costs to be somewhere between $7.05 and $80 quadrillion.
But to address your original point, yes, if Syria were to cost $6 trillion over 40+ years for whatever ridiculous reason, we could very easily afford it.
The Libya intervention is the model here, not Iraq/Ghanistan. Libya is basically an example of why everything the Bush Administration did in Iraq was such an epic fail. Cost-benefit analysis - it's useful!! The problem is that, excluding the Russia factor, the unintended consequences of "success" here are potentially worse than Libya, given the actors and geography.
I still have a Syrian tourist visa in my passport from March 2011, which always causes me to get very strange looks from US customs at the airport.
We could afford the war, but it would be financially inadvisable, regardless. Even so, shrinking from our threats will have costs to American international power which can have downstream costs that are very hard to quantify. It could end up being a penny-wise-pound-foolish sort of thing where choosing not to project American power emboldens other players to come drink our milkshake. It's not a straightforward cost sink if we expect to gain benefits or avoid other costs from the transaction.
