Wool or fleece are both heavy for their warmth compared to down or most synthetics.

I like a mummy bag with a long zipper. I want it to zip almost wide open. That way I use it like a quilt when it is cool and like a mummy bag when it is cold. Synthetics aren't bad and some are pretty reasonably priced.

For 3 season touring at altitudes where it might get cold a 20 F mummy bag works well. I use mine as a quilt way more than I use it as a mummy bag, but when it gets below freezing it is nice to zip up in it.

I managed fine with an $80 Slumberjack Superguide on the Trans America, but we didn't have much cold weather that trip.

If you can count on really mild weather, a bag like the REI Travel Sack +55 sleeping bag is cheap and works OK. I find it fine for colder weather than it is officially rated for. I am fine in it down to at least 40F.

BTW, on the sheet thing... On the trans America we had 95-100F+ degree days, day after day. Three of us bought one queen sized sheet and split it into thirds. Lots of nights that was all we needed and some nights we slept on top of it. I have since been leaving the sheet home to save weight though.