Very cool Phish! Having limited experience sewing with silnylon, I know it's a PITA and takes a lot of work. Kudos on what looks like a great little shelter!
The total cost was right around 100. I used a cheap brother sewing machine my wife bought me Father's Day. The sil wasn't too bad to work with, just have to use a ton of pins in RIGHT spot. I used fielded seams so even though there a ton of extra holes with pinning they are all buried/ covered with the seam. Honestly I have only sewed a locker pocket when I was in middle school and the seams although not factory are functional and basically straight. The math like I said was super easy, but think in 3D. I used a "cat cut" curve with 1.75iin drop over the length of the main seams which I think we're around 6 feet. Let me know if you have any more questions