If you are riding at that temp then money shouldn't be an issue, if it is then stop riding when it is so cold. If you let you feet go numb you are going to get sick.
Not my experience at all. I grew up outside Boston. Went to school in Ann Arbor. Didn't own a car until I was in my 30s and never lived near reliable public transportation. Some of us rode because money was an issue and riding was far cheaper than owning a car. I rode in (then) $30 LL Bean boots and wool socks. (It did cost me needing to send the boots back twice as often for new bottom.) Later I rode with neoprene covers or several plastic bags and creative sock arrangements.
Numb feet and getting sick? I grew up next to a pond. Many days of skating/hockey until too dark to find the puck, slipping my very cold feet into -10C boots and walking on wooden blocks back to the house. Bike's no different. My health held up just fine. Never did get frostbite but I probably got pretty close.
I have gotten softer in my advancing age. I now have the 45North Fasterkats. A true luxury! Now they are rated for -4C (25F). The warmer version (forget the name) goes to -15C (5F).