When someone promises Josh the world, gives him minimally convincing evidence they can give it to him and tells him that he deserves it....Josh will fall for it every single time. It appeals to his ego, his expectations for greatness and his naive nature. Chazz and Scotty have both done this to great lengths, it's always been his achilles heel.
Besides that, he's always been bad with his money. To give Josh credit, he has admitted this and agreed with his father about it. With the shotgun repair, I'd say....you know, if you really loved something you'd probably spend a lot to get it fixed, maybe more than it's worth or more than you should. It doesn't justify it, that price was wildly high and not even close to being worth it but I can kinda sympathize with the idea of fixing up something you love, regardless of the price.