How often are you going? Where are you going? What is your expected low temp?

Three season means something very different depending on where you go - in the Sierra Nevada you should have something reliably warm to 20F and be able to extend that lower in an emergency, because weather will take the temps well below freezing some of the time. How often you go makes a difference - carrying a slightly heavier budget bag a few times a year vs. going every chance you get.

I am out so often that I got myself a 20-25F down quilt, 22 oz, packs to the size of a cantalope, for $240. It is warm enough (I have been out on nights when water freezes in the containers) and compact enough, and it is down, so it will last me not years but decades. I call that a truly cheap bag because I will not end up replacing it if I take care of it. Down bags need careful washing but do not require it often - I've washed the quilt three times and had it for five (going on six) years.

You can find deals on bags, but this is one piece of gear that if you use it all the time, you want exactly what you need - an accurately rated, light, compressible bag. And that will not be CHEAP in the sense that you can find it for a hundred bucks. Pay once, buy well, and it will be a screaming deal ten years from now.