Ac130 models on sketchup are priced at $90+ (the $15 one is adobe illustrator) and if you make a good one and price it low, I am sure people will buy it. It's all about looking around and finding what would be a good deal. An AC-130 is fairly easy to make, just needs good textures.
It is popular now because call of duty has an AC-130 in it, so now everyone knows what it is. If someone were searching for a good ac-130 model, they would find out that that the
decent ones start at $90. If you made a good one and sold it for $5, you would most likely make more money than the guy selling the one for $90.
But then if you search AK-47, you get tons of results, and tons of free ones as well. Alot of the free ones:
http://www.turbosquid.com/FullPreview/Index.cfm/ID/336183http://www.turbosquid.com/3d-models/free-3ds-model-free-highly/252849are really good. There are even better ones that cost lower than $10 and are pro quality. There doesn't really seem to be a market if you make a decent AK-47 unless you make it free and then people begin to recognize your name and browse through your priced stuff.
You can make an M1 Abrams and wind up competing with these
http://www.turbosquid.com/FullPreview/Index.cfm/ID/190990http://www.turbosquid.com/3d-models/free-max-model-abrams-m1a1-tank/483644which are both free and of really good quality (generally the people willing to pay money are looking for great quality and textures as well), so making an M1 Abrams would not be efficient.
I made all this up off the top of my head.