Well, the carbon fiber is ~$30 a yard at 60" wide I think. Then the 2 part epoxy is about $60 a gallon. Most of the car is 5 layers thick.
Best estimates come out to about $1000-$1500 or so. You could do it cheaper if you made some areas less thick and made less mistakes. Usually we put too much glue on any one place, so that gets expensive.
If and when the next car is built, the outside will be only 2 layers thick in most places. Also, we hope to remove all of the aluminum, including the front frame and engine mounts. The only metal will be the steering knuckles, the drum brakes, and the rear hub. Maybe the axles too, but that's still up for discussion. Hopefully we can find or make some tubeless carbon fiber 20" rims.
The over all cost would be in the $2,000-$3000 range if you wanted to do this yourself with less expensive parts. I bet you could get 1000 mpg for $2,000 and make it a daily driver. Certain things will be pointless to spend $xxx on if you gain 5 mpg or even 100 mpg.
In the last 2 years, we have spent ~$11k, which includes a lot of other things, like trip expenses and some shop tools. About $2k went into the EFI engine. We have well over $1000 in rims, tires, hubs, and brakes ON THE CAR. Obviously this would be lower with disc brakes, normal bike tires, and steel rims.
One issue is that some parts may wear out before you use 1 gallon of gas, like the starter pinion, tires, and your body. The ride is pretty rough, but you could make a flexible frame. Top speed would be what ever you feel like, because your drag forces are so low. Top speed would be dictated by the tires, the flimsy body, your nerve to go that fast on 3 wheels, and the time it takes for the clutch to fully engage. I bet 100 mph could be achieved on our 3.5 hp brigs engine. But it would take a long straight to do so. Also, you would have to be confident that the brakes will work....