I have recently started adding base layers to my wardrobe, because it's the first time I've tried biking in colder weather. I've found that running gear works really well and, if you can find it on clearance (which is pretty easy!), it can be less expensive than cycling apparel. I just bought a long sleeve Nike turtleneck running top at my local ****'s Sporting Goods for $10 (clearance!).

On the bottom, I've been wearing my shorts under a pair of Nike running pants (not as tight as tights, but looser than windpants). I'm watching the clearance racks to try and add more pants to the collection.

Good luck!