here's my opinion/statement.
as for the craigslist guy, his materials cost depends on a few things.  good petcocks are 10 bucks, caps, 20+, and bungs, 30+. the steel is the  cheapest, but then add in the cost of the machines to cut out the  shapes, and the welding materials. and paint. too.
he might work at a shop with free use of a welder, gas, welding rods, or  wire and electricity, but that doesn't mean the savings should be  passed on to you.
tanks ain't cheap, and they definitely ain't easy to make. try it yourself and see what happens.
i'm in the process of building my 6th copper tank. materials-wise, it  costs me about 50 bucks, and that's using a high-quality petcock, and  brass fittings from Ace for the cap and bung. 
i put in about 5 hours of hammering, cutting, bending, shaping, and  soldering time today, and 3 hours the other day. i've got the basic  shape together, and i still have another day's worth to install the  petcock and bung, then grind, sand, and polish it.
then another day for it to sit around while the tank liner cures.
this tanks a little trickier, because i'm rounding it all out to fit the  frame better, so there's a lot more work involved, and there are no  templates for a future one, because of the shaping/hammering work. i  started with a flat piece and beat it into a shape.
i've done 2 "toolbox" style tanks that had all flat sides, and those were easier, but still took at least a day.
i have no idea what a coppersmith makes an hour, but whatever it is, it's worth it.
i totally agree with BA's remarks about how people expect the rest of  the parts to be as cheap as the motor kit. it just ain't so. 
look at any other motorsport that uses custom tanks, expansion chambers,  etc. they don't come cheap. the process of building a tank for a  motorcycle is the same for a motorized bicycle. it doesn't cost any less  because it's going on a bicycle. 
if you think about it, it should cost more. if you buy a motorcycle  frame, chances are there's 50 different tanks you can choose from. and  those 50 tanks fit 75% of the aftermarket MC frames.
there's thousands of different bicycles being used out there, and all  those frames are different. to make a production tank for any of them is  a pretty big undertaking, with little reward. 
even if someone were to market a tank for the Cranbrook (arguably the  most popular "entry level" bike used for motorizing) you wouldn't sell  enough to price it comparitively with the engine kit. low cost is the  reason most people buy a Huffy. not too many people would pay 3 times  the cost of the bike for a custom tank for it, and if they did, they'd  end up with a useless tank after their bike falls apart.
hand made, custom products are disappearing, due to the "buy it now"  mentality people have. it's old world craftsmanship in a new world  order.