First and foremost, you have to specify what type of filters you are refering to?
There are a range of filters each with varying functions.
There are UV , CPL , ND filters etc.
It is certainly not mandatory to have but some
are encouraged for the several purposes they serve like to prevent dust and scratches on lens,
or to block out light for long exposure shots in broad daylight. (ND filtes)
Cost wise, it is definitely subjective as different brands have different pricing range. Needless to say, it all roots down to the type, brand, capability of the filters to work out the price.
As a footnote, googling such stuffs will save you alot of time .