It seems to me that aero is the problem here. Being complicated solution, it has taken lot of development money, also huge amounts of carbon parts are spent with each rally. I think i remember some video about TGR Tallinn operation where someone hinted that lot of wings, bumpers and wheel arches are scrapped after every event.

Also rallies should more keep to tarmac and smoother gravel, really rough rallies are expensive as they require somewhat different approach (incl more testing/developing) and break cars more.