With some ingenuity and a bit of scrounging you should be able to gather at least the basics for relatively cheap. You only really need 1 or 2 things from most of these pages, I've just tried to present a number of options.
I'll also note that this isn't really an Instructable, since it doesn't say how to build anything, but it's the best way I could think of to present a relatively detailed tool list for building plate armour. I also opted to use internet images after having a finished instructable, minus pictures, sitting for a month while my digital camera was, and still is, nowhere to be found.
Step 1: 1 knowledge
What kind of armour are you building? plate, maille and cloth armours all use different supplies and techniques.
What is your armour for? is it decoration, costume, or will you fight in it?
If you fight in it what kind of weapons are you using, boffers, rattan, steel?
If your fighting with a group are there any armour standards or regulations you need to conform to?
Also, do you REALLY know what armour looks like? we all have a rough idea, but there is a lot of subtle shapes and details that separate a good suit of combat armour from something to stand in your hallway.
Start building a collection of images of armour, study them closely and really get a feel for the shapes that make up a suit of armour.
Books are also indispensable at the very least i recommend getting your hands on
Techniques of Medieval Armour Reproduction: The 14th Century ISBN#1-58160-098-4
It is the armourers bible, it covers everything you need to know about building armour, and then some. seriously, if you want to build armour get this book, read it and love it.
other than that just about anything you can find on armour will have something useful in it. even if it;s a simple and a picture showing some obscure detail you never noticed.