Got a gift card to REI and combining that with my dividend and the 20% off they're doing now and thinking about getting a new sleeping bag.  

My current one is Army issue, and works well but is bulky.

Looking for something $100 or less.  

I have no idea what I'm looking for, honestly.  I am a side and belly sleeper, and range from a hot to cold sleeper, but err on the side of being a cold sleeper.

Marmot Trestles.  Great bag for the money.

ETA - As a side and belly sleeper myself, you'll really appreciate the additional 1/2 zip on the opposite side with this bag.  
You can get your elbows out of the bag when sleeping on your stomach and still keep your core covered up.

Unfortunately, the only version of that bag that goes below $100 has a comfort rating of 42 degrees and is women-specific whatever that means.  And the weight is about the same as my surplus bag (3.3lbs)  I'll look at it, though, thanks for helping me research.

Marmot Trestles 15 Sleeping Bag - Regular - $81.73
For $100 or under, weight is going to be a sacrifice.  With a compression bag, this goes fairly small.  

https://www.rei.com/rei-garage/product/116071/marmot-trestles-15-sleeping-bag-regular?CAWELAID=120217890003798644&CAGPSPN=pla&CAAGID=16301317480&CATCI=pla-287035862353&cm_mmc=PLA_Google|404_35853|1160710001|none|0465135a-f515-4983-bda4-43040bb90900|pla-287035862353&lsft=cm_mmc:PLA_Google_LIA|404_35853|1160710001|none|0465135a-f515-4983-bda4-43040bb90900|pla-287035862353&gclid=CKKlm_Gs6NICFdYvgQod5yIEhw

Oh awesome! I must have only been looking at their regular site.

Have a Kelty Mistral 20 XL and love it.  Don't know if they still make them or how much...

Look here for some deals
Sleeping bags

Looks like a hell of deal!

Buy a bag rated colder than you think you need, but dont get silly.

20 deg nights, get a 0 deg bag.

Dont go to bed while salty....use baby eipea to clean the salty sweat off your body.  It is night and day more comfortable.

If its really damn cold, go to bed with less clorhes on.  Seriously.  

Use a sleeping pad.  The ground has essentially unlimites heat sinking ability.  If you put your bag on the geound with no pad, you will be cold no matter what.

Keep it dry.

Mak sure it is sized for you.  You want to be gently wrapped, not snug.  You dont want your feet wedged against the bottom.  The more distance you have between the inside wall and outside wall of the bag, the warmer you will be.

If im not looking for something lightweight/technical, just buy what suits your needs and budget.  

Klymit 20F..Amazon 135.00 dry down

Lots of bags out there.
I'm impressed with my daughters (we just got her to last her into her teens) 40f rated dri down from slumberjack..under 3lbs..got it off woot..so price was well under 79$.

On the Marmot.

I've owned the 32f trestles. Since 2010ish.
Great bag. Good fit.
Temp rating is on spot imho. I've had mine into the 20something. .with little to no issues.
Its a little heavier but it packs up pretty small.