Exactly. 340 has 22 extra cubes and better heads, intake, cam, rods and exhaust manifolds, but you can't find a cheap 340 any more (I'm old enough to remember when you could). 318's, on the other hand - well, one of my Studebaker friends ended up with a 70 Fury with what appeared to be a complete, probably good running 318. He thought he'd be able to sell the motor for a few $$$. Couldn't even give it away. Finally just scrapped the whole car.
If you don't already have either motor, then the 318 is so much cheaper to get into than a 340 that it makes more sense to start with the 318 - which was the point of this discussion.
On the other hand, if you already have a 340 and a 318, starting with the 340 makes more sense. Dollar for dollar, you'll get more out of the 340. Especially if you're on a budget, because it's not just the extra 22 cubes, but also that with a 340 you're starting with better intake, heads, cam, etc.
But it's fair to point out that 360's are still cheap, so unless I already had a good 318 sitting around, I'd start with that.