I think you have to look at it from a different angle. If you have an idea of what your next project will look like as an end result and you can find a similar new bike to start with then just start adding up the cost of individual parts. I have, in the past, cut up brand new bikes to build new cruisers. If I'm planning on using the wheels, brakes, cranks, pedals, bottom bracket, or forks for the project, then the cost of those components alone almost always add up to more than the complete new bike. I can then sell the frame or cut it up and uses pieces of it for the new custom. Believe me a threaded bottom bracket tube or a neck tube can cost quite a bit unless you want to buy 25 at a time from a frame builder.
it's important to be specific to pick a bike made from aluminum or steel depending on your abilities. I've seen, and made myself, some radical bikes from aluminum but it can be a difficult material to work with without the correct tools or skill level. Steel being easier to weld and fabricate, seems to be the reason most choose a steel bike as a starter.
If you start with an old junker and build a new bike out of it, that's cool too. I've gotten old "barn finds" and started working on them only to find out it was a rare prewar skip tooth balloon bike. So back into the pile it went or off to my bro's house cause he restores bikes and sells them for a killing.
My favorite "new bike buy" was about five years ago when Kmart in Denver, CO was discounting all of their "OCC" Schwinn Stingrays cause they just didn't sell enough of them. I bought (10) bikes at $29.97 each and cut them all up. I saved only the wheels/tires, forks, handlebars/neck, neck tube, BB and cranks. The rest got chopped up and taken to the trash! Those bikes listed new for $199.97 each. That's one heck of a markdown. 20 inchers don't really interest me much but I figured I'd hang on to the parts for a future project. I'm planning on building some smaller 20" fat tired mini-stretch cruisers for girls and kids based on my full sized bikes. Those parts would take care of 5 bikes right there with minimal investment of my own.
So yeah if you want to start with a new bike you can, just make it your own, and own up to it!
Later Travis