Yep, what amk said. $$$$
Most people that build a jet john the first time out aren't into spending a lot of money to get a running boat. They may spend the money on fabricating decking or other structures, and everything else that building a boat entails, but most people are looking for a relatively inexpensive, lightweight engine with moderate power.
When I built my boat, I used a 600 dollar Tigershark engine, because, for one thing, I wasn't sure if my idea was even going to work, or how it might perform. But after having had the boat and run it for a while, I was convinced it needed a more powerful and reliable engine, especially with all the problems it was continuing to have with blowing holes in the pistons.
So, I bit the bullet, and spent 2500 dollars on a Yamaha MR-1 that I got for a really good deal, considering the low hours. It was a lot of work to re-configure my engine compartment for the new engine, but in the end, it all worked out great, and I just wish I'd done it this way to begin with!