i wear midweight smartwool bottoms (over my bike shorts) and then a pair of mountain harware tech pants I scored on sierra trading post for about $70 for BOTH. if it gets warm, i slip the long johns off.
the tech pants are awesome, as they stretch in the back and they do a fair job of keeping the wind off of me in the front. my buns do get cold (while in the 20 and below), but being what they are (my buns), they work as designed and with little bother. anything above 40 deg F, I omit the long johns.