Instead of warming the entire room you can get a heat mat or heated propagator and just warm the seed trays.

The ambient air temperature still has to be addressed...30 degrees F is too cold. There's also the issue of seedlings hating their roots to be overheated once they've germinated. I unplug my germination flats prior to transplanting into cell packs and maintain the growing plants at around 50-60F.

Sam, describe your set up....method of growing, how many, types of containers, etc.

I have enclosed a section in my unheated garage for my little growing operation. I have three large plant carts which have lights above each shelf and place a little heater on the floor. I even have a little circulating fan hooked up.

It can be in the teens outside, but between the lights and the heater, it stays warm enough inside that enclosure.

You may want to consider an indoor enclosure, too. Click HERE for ideas.

I dont have a setup at the moment. We moved in the summer so I have nothing set up. The sunroom will most likely be too cold(and heating the whole room would be a tiny bit expensive) I am considering doing it in my bedroom. I have a wooden plant stand that I was hoping to use. I plan on starting around 100-150 seedlings this year.

This doesn't address the keep-it-warm question, but here I go with the fish tank thing again. If you need to deter cats, you can enclose the seedlings indoors with something as simple as a transparent glass box, I.E. a large fish tank with some sort of lid. You could also make the equivalent cat-free shelter with pieces of glass or Plexiglas and tape, or a six-ish foot long piece of hardware cloth, say 18" wide, bent into a rectangular cage.

Yes - I jumped in a bit quick there. I was just focussing on the germination and not really registering the sub-freezing room temperature. It would useful to know what plants you wish to grow as some will need higher temps than others.

100 - 150 seedlings could take up a lot of room as they get bigger and need pricking out and potting on.

I'm going to be starting Peppers, Tomatoes, Onions, possibly peanuts, and some flowers