The shop guys are right - there are some pretty flimsy looking winter jackets that are incredibly warm. I was skeptical myself until I showed up at a small ride bundled up like the Michelin man and one of the other guys (a pro at the time) had just a jacket and a thin baselayer. Since we teased him relentlessly about always bundling up, to the point of wearing long pants in 70 or 80 deg weather (our exact phrase was "it's pro to be warm"), I asked him about the jacket. He told me he's good down to stupid temps in the jacket. Eventually I bought one like it and he was right, the jacket was fine down to stupid temps (for me that's 15 deg or so, although I've ventured out in colder temps if I'm riding on trails, like -10 to 0 deg).

Jackets do three things - they block wind on the (front of the) arms and torso; they provide some insulation; they offer some ventilation in the back. Because you're riding you'll be generating some of your own heat so the insulation needs to be minimal.

To replicate a jacket's function without dropping serious coin you can use some tricks. These work down to about 35-40 deg.

1. Put a plastic bag or newspaper under the front of your outermost jersey/top. If you're getting cold you can do the same on your arms. This provides wind protection. I found plastic bags better than newspaper because the newspaper ink usually runs a bit.

2. Wear layers. A base layer and then insulating ones. Fleece is fine for insulating, whatever holds air. Remember that you'll cover everything with a "kit" outer layer like a LS jersey or SS jersey with arm warmers. Remember the wind protection that goes under that outer layer either using plastic bags or...

3. Wear a windproof jacket over everything, or a vest if it's not too cold (but with a vest you need to make sure your arms are warm enough). I hate those wind jackets because they catch so much wind, the inflate once you get going. However they work if you can't get a jacket just yet. You can even use one of those clear rain jackets, it's windproof but won't breathe.

You'll find that the jacket is not your limiter. It's your hands, feet, head, neck, and groin. Get reasonable gloves (I seem to be okay with the thinnest gloves I can find rated to 25 deg F or so, much thinner than the lobster type gloves), booties (I use Sidetraks, they're very inexpensive and last literally 10 years or more; most zipper type booties will last a couple seasons), some head and neck tube thing, and windproof (key for colder weather, for me that's about 32 deg or colder) tights.

In crazy cold temps my torso is almost always warm with a jacket, 1 long sleeve base, 1 long or short sleeve jersey. It's my legs/groin and face (my chin goes numb first, then my nose) that get cold.