Favorite double sleeping bag for backpacking?

Best combo of cheap, warm (down to maybe 20-30*F), and lightweight/small pack size. I realize that's something where you probably only get to pick 2... but just let me know what you like!

Check out the Big Agnes Sentinel 30 and King Solomon 15. Both are fairly warm/decently light and packable (650 fill), and both go on sale fairly often. The only downside is the lack of down on the pad side of the bag (not too big of an issue if you have a good pad). There are warmer, lighter, and better bags available if you want more, but they seem to do the trick for me.

You do in fact only get to pick two. Keep in mind that most backpacking bags come in male and female versions. The female versions tend to be shorter (66" regular length and 72" long, vs 72" regular and 78" long for mens), have less shoulder room, and I believe the insulation is distributed differently. Most shops stock them with the zipper on one side for the men's version and the zipper on the other side for the women's version. Some bags can be ordered with the zipper on either side for men's or women's versions, in case you want to zip two men's or two women's bags together. What I'm saying is, don't get one double bag, get 2 single bags that you can zip together. That way when you're traveling alone for any reason (or just pissed off at each other), you've each got a single bag. This also lets each party choose the right length and amount of insulation.

Define "warm". What kind of overnight temperatures do you expect to encounter? 

For synthetic bags, the various Marmot Trestles Eco Elite bags are good value for the money. They're lighter and pack down smaller than most synthetic bags of comparable temperature rating. Sea to Summit also makes some excellent synthetic bags. 

For down bags, the REI Magma bags are excellent value for the money. Cheap, low fill power down bags tend to lose their loft much more quickly than bags made with higher quality down, IMO.

My first double bag was a Big Agnes King Sol (15*) that got a lot of use and was worth the weight as long as the rest of the kit was distributed evenly with my partner.  The BA bag is still in use when we go car camping.  The lack of insulation on the backside isn’t an issue because that side shouldn’t rotate away from the pad and compressed down doesn’t have much insulation value, the pad is your insulation against the ground.

I’ve since moved on to a Feathered Friends Flicker that is much more versatile, half the weight, packs down well, but requires more discipline to stay warm.  My partner and I will take 3x double length slings, give them a twist in the middle to form two loops, put them over our sleeping pads to keep them aligned and gap-free and then clip the carabiners to loops on either side of the bag to keep the edges down.  We put a fold in the foot box to wrap all the way around our heels.  The part that takes discipline is not lifting up a leg or arm in a way that draws air into the bag (now a quilt really).  We’ve slept comfortably that way down to ~20* and I’ve used it as a single bag for just myself a lot in the mid-20s.  All of our use with this bag has actually been on snow and it‘s a system that works well for us while staying light.  We share a 2lb bag, we each have a 1lb inflatable pad, and we share whatever tent is appropriate for conditions.