1.Permanent retention  is required to hold your tooth in the same position once you have finished aligning your teeth .. i.e. after orthodontic treatment . The treatment cost varies in each dental clinic.there are many ways of retention the most common is either a removable plate or fixed retainers... 2.Removable plate is easy to remove and oral hygiene maintainence is easier with such plates but the disadvantage is its difficulty with patient compliance 3.fixed retainers are good only thing is difficulty in using a dental floss in the interdental areas in between the teeth but that can be managed with timely cleaning which the dentist would do for you

U cannot have uniform price throughout India as it's not a product. Being a service part different doctors charges differently. If it's properly done there won't be any disadvantages with it.