In simple terms, a record track is a wiggly line. Bass frequencies make the line wiggle more violently than treble frequencies, so those frequencies are reduced (attenuated) when cutting the master.

This means that the track density (playing time) can be increased, but it also means the stylus will track better, damaging the disc less (each playing damages the disc, no matter how good your equipment!).

When the record is played, the attenuated frequencies must be restored, which is the purpose of the RIAA equalizer. Most RIAA equalizers also include a level raising pre-amplifier for magnetic cartridges; the output of ceramic cartridges is significantly higher, so they don't need a pre-amp.

Price? Difficult to say, as they aren't readily available any more. When they were commonplace, cheap battery powered ones were around 5 pounds sterling (around 8 dollars), but mains powered ones were almost ten times that.

As I said, any domestic amplifier designed to reproduce vinyl will already have the equalizer; otherwise, a 'disco' mixer is another option.

If you play a record without EQ, it will sound thin and nasty!