Some people build custom bikes because they are cheaper than buying one. Others just want the I-built-that feeling as they look at it. Still others might want customization that they can not get in an "off the shelf" model.
When someone decides to build a motorcycle, there are three paths to take depending on budget, mechanical skill, design aims, and past building experience. Those paths are custom bike kits, a rolling chassis, and building the whole thing from a collection of motorcycle parts.
Custom Bike Kits
The simplest choice for someone who has never built a bike before is to try one of the many custom bike kits out there. You get a box that has everything you need to build the bike from the ground up except the paint.
Kits are great for first-time builders. If you can follow instructions, you end up with a bike put together with your own hands. You do not have to worry about picking the wrong parts or whether two pieces will work together. You will make mistakes along the way, but they will tend to be safe mistakes that will not ruin the build.
The biggest downside is the cost. This is perhaps the most expensive way to build a custom bike. You are also limited as to how much customization you can do. Sure you can swap out the included parts with others of your choice, but that means throwing away pieces you've already paid for.
A rolling chassis is a partially assembled bike that you can customize. It includes the frame, handlebars, wheels, gas tank, and other basic parts. You need to add an engine and a transmission then finish it off with custom pieces.
This is a good middle-of-the-road choice. A rolling chassis is cheaper than one of the kit bikes while keeping the advantage of pre-selected parts that work together. Since all you have is the undering framework, you still have lots of customization options.
Building a bike from a rolling chassis requires more skill on your part, since you are going to have to choose the other parts yourself.
A variation on the rolling chassis is to take an existing bike, cut it down to the frame, so producing your own chassis. A used bike will probably cost less than a rolling chassis and you will be removing the parts that have seen the most wear and tear such as the engine.
Building from Scratch
The ultimate in a custom bike is one that you select all the motorcycle parts yourself. The bike becomes a true expression of your personality because every nut and bolt was hand selected. Needless to say this build is beyond most first-timers and even most second-timers.
You can either go cheap, cannibalizing parts from junk bikes, or expensive, buying high end custom chopper parts. This is definitely a blank canvas and the ultimate challenge for the bike builder.