It's been asked before here and many other places; touring expenses is a topic that's easily researched.
As noted, too many variables. You've been presented a ten-to-one range of <10 to >100 US$/day. That's impossible to reconcile but it is easy to average or find a median which you are comfortable planning for. Let's say $30/day for 70 days (which is spartan and depraved but entirely doable and, from what we hear around here, possibly pleasurable)--$2,100--but call it $3,000. If you budget for $60/day (which includes camping mostly, cooking mostly, layover or storm days in motels and an occasional restaurant, that's $4,000-5,000.
Regarding the idea that you're not spending money at home, you may be paying your rent or mortgage while on the road, paying someone to house- or pet-sit and, if you're leaving family behind, they're still eating and driving the car around. ONe trick is to find someone who will pay you to stay in your home while you're on your bicycle tour. Dicey, that.
Visit your local library and find everything you can on bicycle travel. The vicarious experience is a fun way to spend your winter and you can learn much from mistakes already made. The biggest hurdle to a long bicycle tour is simply committing. ONce you've decided you're going, planning is mechanical.