I don't believe there are many websites out there that provide cage building tools...
The easiest way to go about it is to set your mind on what size cage you would like....
Then, get some graph paper, a ruler, lots of pencils and erasers, and compass sometimes helps...
Draw out your first rough draft of all sides.. One side per page of graph paper.
I went through 87 pages of graph paper before I was settled on my design. Things were constantly changing.. Do I want a glass front? Do I want the cage to open from the front with doors, open vertically from the front, or do I want the top to raise up?
I can't give you a price on how much building the cage is going to cost because I haven't gotten as far to purchase all the materials... This is where my father comes in.. Him and his Home Depot contractor card.... 
But anyway... The largest non-special order plywood is 4x8, they range from $15-$45 depending on thickness and quality. You will need 2x4's, these act as braces and supports. I require about 150 feet of 2x4's....
Then you have to decide on how you will be doing your heating.. The easiest way is with clamp lamps, but if you want a secure way, hardwire the lights. Then you have to decide on front, do you want a viewing window in front? If yes, will it be plexi-glass or glass? Is the top going to open or is the side going to open? Will the cage be on casters or sit directly onto the ground? What are you going to waterproof the inside of the cage with? What decorations will you use? How will you supply freshwater everyday? What will your substrate be? Dirt, Mulch, etc? Where do you plan on putting the cage? How will you fill and clean the cage?
Those are just some of the questions I had to ask myself... Some were easy to handle, others had to be worked out. The best way to start is to start drawing out your sketches of the cage... They will change everyday, or at least they did for me... Also, you want to build the cage as big as economically feasible.. The reason for this is because it will cost more in the long run if you're constantly having to upgrade the size... Make it full size to start, save money in the end(if that's possible)...
It took me nearly 3 weeks to plan out my cage.. I figure it will take me and my father at least 1-2 days to complete the cage, half a day to hardwire the lighting/heating, a quarter day to get the pond, substrate, and pond setup. Then another 2-3 days to get the temperatures worked out right.
Good luck with your decision! Building your own cage is a challenge, but worth it in the end. I've never built a cage, but have built acrylic fish tanks. They are like comparing apples to oranges, but once you do it, you can take pride in your creation and you'll want to do it more!