A good RO system will remove 98% of the TDS so 220ppm will be reduced to 5ppm so it’ll cost about £1 per 1000l in resin to polish it off.
To produce 1000l of pure will use 2000l from the tap, which will cost around £6 I think.
A combined prefilter costs £20 and will filter 37500l pre RO, to produce 18,750l of pure so around £1.10 per 1000l
So £7.10 per 1000l or £0.007 a litre.