Rapha, nice but a bit pricey?

i fancy one of those, just have a hard time swallowing the price tag.

would you say it's wind-cheatingness is enough that, if paired with a warm winter jersey ( rapha, natch) and base-layer, it might negate the use of a winter jacket, on all but the coldest days?

you don't need a warm winter jersey with it imho unless it's long sleeve and you omit the base layer.
i run quite warm so it needs to be below 8-9° for me to wear it.
usually rapha base+jersey underneath.
i need to get some thin armwarmers so i can just have a jersey on underneath to use the pockets, when commuting i just have the base+jacket as i don't want pockets under my bag.
if i'm brutally honest you could get a pertex windshirt type thing instead but:

it wouldn't have the cut/fit of the rapha, nice high collar
the neat drawstrings
the orange colour and reflectives which mean you don't look like a fluro yellow nodder or a blue altura fan.

i'm going to wear it for most of my winter riding and can't see me needing more than roubaix bibs/merino base/jersey underneath it. i have a thicker jersey if it's just above freezing.

what has impressed me is how well it deals with a bit of rain.
if you feel like treating yourself then you will not be disappointed. if it leaves you with no spare cash to buy other winter riding essentials then think again.