Been using a Foredom flex-shaft for 50+ years and also have an air-driven hyper speed unit (400,000+ rpm). Burrs, whether steel or abrasive (carbide/diamond/ruby,etc) can get expensive until you settle on what you need. There is, however, a really cheap way to go: ask your dentist! Most dental burrs are (or were) single use items because of the sterilization requirements. If you catch them at the right time, they might just give you a bunch before they throw them away.