Sounds like the original poster is making their way South and should in "Florida cold" soon instead of "Chesapeake cold." Fortunately they didn't start with "New England
Some thoughts of my own, based on deliveries when I can't build in a lot of kit to the boat.
The flower pot on the stove isn't useful - burns cooking fuel
and dumps water into the boat. Making tea and chicken broth is better - more heat into the boat, same water into the boat (bummer), and drinking it warms you up.
Zippo (the lighter people) still make hand warmers. The cost break-even point happens real fast compared to the chemical throw-away bits. You can get lighter fluid anywhere and it's real cheap
Really good underwear is the bomb. I use Damart stuff, although a little hard to get in the US these days. This is the kit Chicago motorcycle police use in winter. Worth every penny.
Have two or three sets of warmest clothing
so you can rotate through and allow it to dry.
Take a layer off BEFORE you go do something that might break a sweat.
Two pairs of ski gloves (or better mittens) per crew is another good investment.
You've got to be able to see. You've got to keep the wind off. No good answer to both those. Do your best.
Motorcycle heated gear
with more insulated clothing
on top beats the heck out of electric
blankets or other kludges.
Pull back the bedding when no one is in the berth - you want to give the fabric
every chance to dry.
I have some other heat on my boat, but the original question was for lowest cost. Nice to have a warm dry boat while iced in on Back Creek in Annapolis
. Low was 16F last night and the boat stayed dry and in the mid-70s!