Wood prices SUCK here in the states right now, but you gotta do what ya gotta do. 
Here one idea for the pondering...
Use the cheapest 8x4 boards you can find for the internal walls. It helps if you build the frame on the OUTSIDE, so the inside has 6 smooth walls and no "skeleton" to get in the way of cage props and internal finishing, such as frp pannels etc. This also makes sealing inner edges simpler! Over the ouside to make it styleish, you can get a better grade of wooden panelling to lay over your external frame. This outside siding can be stained and embellished with decorative, carved edging pieces to add to the asthetic appeal of the finnished product. You can find commercially constructed sliding windows in various demensions to use as the front doors of your enclosure.
I believe that John A has some sketches of this type of cage design over at cybersalvator.com ~Perhaps that will aide you better visually.
Good luck! Keep us posted on how it turns out! 
~Michelle