Air Water Ice makes alot of units in the $100-200 price range. Their Typhoon III RO/DI is highly regarded in this message board community and comes complete with things like a TDS meter.
Some should be changed every 6-8 mos, others every couple yrs [the membrane]. You need to get a TDS meter and test your water when you do maitanence.....when the meter starts to show pollution coming thru the filter into your water.......time to change the filter.