Try to buy some Chinese bits ,a dozen of them in different size there is no way to avoid braking some of them, was In procedure of eliminate some router bits before I sell my previous CNC,lol.
No matter what the calculators says about speed and feed it also has to to with your machine's settings as Davek already said. So better to brake some cheap bits than to see to cry for the expensive ones....
I can't recall how many times I stamp a bit in a wood and Plexi because I forgot to zero Z axis!!! ,:-P