All horses should have their teeth examined at least yearly, most will need to be floated at that time, but some need it done more often and some can go longer between floats. By the time they're dropping food and showing discomfort from a bit they're way overdue.
The cost can vary significantly even in the same area based on the practitioner's experience and technique. Some will charge based on the severity of the points/ramps/waves/etc they're correcting. Some float manually, some use power tools. Most horses need to be sedated for the procedure to be done thoroughly, and in some states a licensed vet is required to administer it (you will find some unscrupulous "equine dentists" who do not have a DVM and administer sedative illegally)
I use a vet who no longer does general practice, but specializes in dental, acupuncture and saddle fitting. She charges a flat rate that covers power floating and as much sedative as needed.
I'd ask your normal vet if she does floats and/or if she knows a specialist that she recommends. At the very least, your regular vet should be able to examine the teeth to tell you if they need floating.