vince,
RO and RO/DI units range in price from about $100 bucks for a simple RO low gpd gallons per day - to well over $300 bucks for a full RO/DI system with much higher gpd output and prefilters.
There is a ton of good info on the net - do a search on Reverse Osmosis and you'll be swamped with helpful information.
Things to consider;
1) What are you wanting to remove from your freshwater source, and to what degree removal.
2) GPD output of the unit
3) RO only
4) RO/DI combination
5) Installation
6) Cost of replacement RO membranes and or Deionization mixed bed resin cartridges.
7) Cost of the entire unit
There are several major manufacturers out there that all make great equipment. Usually you'll find the best prices buying online. You'll be glad you got one
HTH - Brian