I don't know is the answer but if that is the system that puts vents in ceilings of a few rooms and pushes down air I had that once for first line of radon mitigation (it didn't work well enough and had to come out and sump fitted). Anyways can't even remember cost as they allowed it against the full job when it didn't work. However I would say be very careful where you site them, the bedroom one used to drive me mad blowing air that I could feel on my face when in bed, added to that the constant hum of the fan in the attic also irritated me. I was never so glad something didn't work right and have to come out!
I agree a fine big dehumidifier would be a good start. Is the problem in every room or just some, bought one recently for my daughter where she has a problem in one end of the house and it has almost instantly stopped the returning mould patch on the bedroom and bathroom walls. Worth a try, I paid I think 220 for Meaco brand which was recommended to me, got a 10l but it comes in 20l too.