I tried designing a backyard coaster once too. Calculating costs, it came out to be $355,000+ just for the fabrication of the track
. Still had to factor in supports, magnetics/chain, railings if needed, and trains of course. I actually had a whole plan set up for future use, found a good steel fabricator, found a very reputable magnetics company, etc. But once I saw the costs add up I knew it would never happen haha
Good luck though. It's really a lot of fun struggling to design a good, fun, and safe coaster that fits into your required plot of land. You get to see a little bit of what real designers have to go through to make their coasters a reality. It's definitely one of the more interesting concept's I've designed.