When you say as an extra layer, do you mean something to sling over the top of everything else (including possibly a wet waterproof) for extra warmth at a lunch stop or an extra mid-layer to wear while walking?
For the former in winter I carry a reversible jacket made by Berg and stuffed with lots of Primaloft Silver insulation that I picked up cheap from Sports Pursuit about 4 years ago. The outer withstands light drizzle but even if it gets soaked it still adds useful extra warmth. Since I bought it the trend seems to be for new fancy synthetics that mimic down more closely by being in lots of little bits, but this has a sheet of insulation inside it. It's not as warm as it was, I think the insulation has degraded somewhat, but it's still useful. Lots of companies do this sort of jacket and as they're not the in thing they tend not to be too pricey.
For a mid-layer I love my Rab Alpha Flux jacket, which again I picked up in a sale. Not at all sure it would be worth the full price but I do find it works brilliantly at wicking the sweat away and keeping me just the right temperature as I go up and down hill. It's a snug fit, in summer I wear it over a wicking t-shirt, in winter over a thermal base layer. I wore it in the Brecon Beacons in the snow over a thermal base layer and that was warm enough while I was walking uphill. When I need more, I add my waterproof or softshell, and if I'm stopped I add the insulated coat.