The cheapest way to get into bamboo is to "find" one in a garage sale, or an antique store, or even ask your grandpa if he has one. You may not find one for quite some time, but eventually something may come along.
Another way to get into bamboo is to purchase a restoration project. There are some collectors who do not have the time to "bring a rod back" to its original state. They sell the rod for a fraction of what it could be worth. You supply the elbow grease and you have a nice rod inexpensively.
A step up is to build your bamboo rod from a bamboo blank. You can purchase a blank from a company that caters to this type of rod building. Here you have a bit more control over the type of rod action, length, line weight and price. You will probably spend around 500 dollars or more, putting it together. But now, you have a custom bamboo rod that you have built yourself!
Ok, you want to get a custom bamboo fly rod? There are "Modern" bamboo rod makers that will make you a rod. Either to your specifications or a model that they build and recommend to you. I have purchased a modern fly rod from a rod builder of my choice. I believe that you can find a fly rod exactly like you want if you work with one of these craftsman.
There is a wealth of information on the web to help you with any and all questions you may have about building your own bamboo fly rod. Now, if you break your rod, you know how to fix it. You can make a rod that suits your needs exactly, and you will probably enjoy the time spent through the different stages of building. When everything comes together, catching your first fish on your own bamboo fly rod will be a monumental event.