Hot take: sleeping pad R-values are mostly a sales pitch, and I'm done paying for them
I camped 14 nights in the Adirondacks in April with a cheap closed-cell foam pad that cost me $15, and I slept fine the whole trip. People keep saying you need an R-value of 4 or higher below freezing, but you don't get cold from the ground alone, you get cold because your bag's bottom is squished flat and does nothing. I added a reflextix sheet under the foam and my back never got cold even at 28 degrees. The pad companies are selling you a number instead of teaching you how the whole sleep system works together. What's the cheapest setup you've actually trusted in below-freezing weather?