I would not say wfp running costs are cheap at all compared to trad, you have resin, filters, pole hose hose reel, connectors, then there is the rectus 21 fittings, not to meantion the poles and a few others thing I am sure I have missed.
Please dont get me wrong, I love wfp far safer than trad, but I am being 100% honest, wfp does cost money to maintain, the price it costs will depend on the amount of work you do, the amount of water you use, and the tds level of your tap water and if you are on a meter or not.
Ian