I guess we need to redefine where we think hydrocarbons come from, http://www.cnn.com/2008/TECH/space/07/31/cassini.titan.ap/index.html?iref=mpstoryview
There's all the fuel we need, how do we get it?
If you think gas is expensive now let them try this. It will get even more expensive.
I found one site that estimated the average cost of space launch was about $22,000 per kilogram. That would be the cost of getting what ever space craft off the ground here.
It took the Cassini-Huygens spacecraft almost 7 years to get there
$3.26 billion was the total cost of the mission and that spacecraft only weighed in at 2500 kilograms. I am sure any vehicle would need to be vastly bigger than thay thing was to get enough fuel back here to be worth the 14 year round trip.
There is also the much bigger cost and it is not in dollars. The cost to our planet in the amount carbon that would be released into the atmosphere. The carbon being released now was at one time in the atmosphere or on the surface and it is causing problems now, imagine adding millions of tons more carbon that was not even here in the first place.
Of course as I usually do I am probably blowing this out of proportion, after putting all the time into some of the research I came to realize that there is the possiblity that you were joking. However I had done the work so I figured I'd post anyway in case anybody did think that this was a good idea.