I think you are going the wrong way with the JGRO design if you want to stretch it to 4x8. I just finished a slighty stretched JGRO (cutting area of 2x3) and with the unsupported rails I have some flex. I have plans to fix it but they wouldn't make sense on a machine much larger than this one. Over 8' I suspect that the bending of the pipes would make the machine very inaccurate. You also have worry alot more about leadscrew whip and racking when you go longer.
For budget I spent about $150 on wood and pipe, and angle. I spent $300 on motors and controller, I spent another $100 on bolts, nuts, lead screws, and bearings. There is probably another $50 in other things I am forgetting about. So that puts my cost around $600. If you want to upscale the machine your costs will be exponential. The difference between a 6' acme lead screw and a 10' one is quite a bit. If you want a full 8' cutting area you will have to make the machine longer than 8' since the distance between the router tip and the back side of the gantry limits your travel. The JGRO is a great machine if you build and use it within its limits. If I were you I would look into a joes hybrid machine. It would be much easier to upscale to 8' long and fixes some of the shortcomings of the JGRO.