As it happens I have some experience with using and sourcing coated camera optics. A few observations: you get what you pay for to some extent, re-coating a camera lens involves a certain amount of labour then some time on fairly expensive equipment (vacuum coater) operated by a skilled (you hope) technician. Parts have to be cleaned rigourously before cleaning and probably inspected than packed afterwards. It all costs money.
Originally Posted by JPD
Coating type: MgF may be fine for the inner surfaces that are not normally able to be touched but for the first surface you have to consider the coating durability. However good it is optically if it wipes off the first time you try and clean it that is no good. So ask any potential supplier if they are applying a hard coating and if so how they define "hard". I have definitely seen very soft MgF coatings - they will be partly removed by just one pass with a paper wipe.
There is really no optical downside to coating, it wll not affect sharpness as someone implied above - although disassembling the lens to coat and reassembling slightly off obviously could.