Robbos are 'cheap' up-front because they're built for a 12-year / 2,200 flying-hour service life; they're basically disposable helicopters. They can be overhauled and rebuilt at that point, and every subsequent 2,200 hours, but it's expensive.

Furthermore here are some service-life-hours comparisons between components on an R22 and an Enstrom F-28C of the same category ( edit: updated from the FAA TCDS ):

  Main rotor blades: 2,200 / unlimited on-condition
  Main rotor spindle:  4,400 / 17,700
  Tail rotor blade: 2,200 / 3,100
  Tail rotor attachment:  2,200 / 6,200

Source: an acquaintance who runs a training school with R22s. He loves flying them but sells them on ASAP when they're anywhere near their service life.

Incidentally Mr Robinson originally designed the R22 with a target list-price of $22,000, hence the name.