It takes 2 ro filters and a carbon filter. Both ro filters have to be changed at the same time and if you change them, you must change the carbon filter. In other words, all 3 have to be changed.
Cost is about the £150 mark and I've found the difference between different suppliers to be just a few pounds.
As for running costs and efficiency, I don't personally know as I don't pay for water, yet.
One thing with a merlin though is the higher the pressure, the better it works.
TDS output isn't wonderfull at about 15 - 21 with new filters. I changed mine about 2 months back and the output is now about 35 at approx 200 ltrs ave per day.
The main benifits of a merlin is gpd. I fill my van in a car park each night so for me, how quick it fills up the tank and barrels matters. 25ltrs with new filters and good pressure takes about 15 minutes.
If the speed at which you need to produce water doesn't matter, use a 4040 or something else. You'll save money on resin.