A problem with most diamond burrs is they are electroplated onto
ordinary steel and the quality control is almost nonexistent at the
cheaper end. Whether they last is often a matter of luck but I tend
to buy them as cheap as possible by the 10+ and treat them as
disposable. You can get them for pennies each from India, Pakistan
and China and as long as you buy a box full the postage isnt too bad.
Other than that you need to find a company that does proper sintered
bits and these are getting harder to find as the plating method
means that it is easier to get an individual shape made at a low
cost. Hav a word with your dentist and ask them what suppliers they
use as dental bits used to be better than those aimed at the hobbyist
with a finer particle size and proper sinter coatings
Nick