You are right on the cost. When I bought the water filter (about $30) it was because I could come up with $30 at the time and couldn't the $130 approx w/ship for a distiller. The water filter came with a filter and I am already needing to replace it. Can't remember how much the filters are, about $10 I think & I am thinking I would need to replace it about every 1-2 months, so in the long run--definitely not cheaper. The water filter was not intended as a replacement for the distiller, just a band-aid until I could get one.
I live in the Oklahoma panhandle and we don't get a lot of rain but might be worth looking into.