This skull cap fits the bill for what a winter under cap should do. It retains heat remarkably well, It stays put and doesn't ride up. It is lightweight and comfortable.
I use it in cold and very cold weather under an insulating hat and not a helmet. I have also taken to using it off the bike too in combination with another hat and sometimes alone.
I bought it on sale, so it was an excellent value. I am not sure what the normal retail is, so that calculation would have to be made.
This Pearl Izumi Barrier Skull Cap has a nice snug fit to it. I especially like the lower sides that fully cover exposed ears! There is a ponytail cutout in the back that I have no use for, maybe if my hair comes back I'll be able to make use of that? I wear this for any ride below 50F, down to about 30F. Below that I switch to a balaclava.