The difference has to do with the craftsmanship and quality. For example, professional trumpets are designed for specific needs, like particular music. Jazz musicians might, for example, invest in a professional trumpet which helps them reach higher notes with more precision or project more efficiently for solo performances. Beginner trumpets are designed to be versatile and work for all purposes. They work for all types of music and all types of musical groups. They are not as specialized in their design, but that is why they are less expensive.