So I have three totes with little rows of lettuce/Swiss chard/salad mix in each one. They're only about a week and a half old, so they're still fragile but have cotyledon and are working on the first true leaves. BUT it got colder than I was expecting and I don't really have a place to warm up the plants AND get them access to warmth. How do I heat my plants where they are by spending the least money possible?
I think you don't need to warm up them, just protect them from cold weather:
you can move them near a south wall: now the sun shine most of the time, so a wall will sto heat and give it back on the shorter nights
you can store them in the garage at night
you can build a small temporary wood wall in the direction of predominant wind (if you are in a windy place).
Some plastic cover or hay could also help, maybe two layers: one on the ground for humidity and roots, and one on the cool days (or only on the night) on top of plant. If you use it only on night or very dark days, a non-transparent cover is better. This will help keeping the stored heat.
Since the seeds have already germinated, I would be less worried about heat and focus on getting the plants enough light. You can move them closer to a window so they get light during the day. Alternatively, you may want to consider taking a lamp from some other location and putting it close to the plants so they get some light.