It does depend- how long will you be inactive during the day and how long will you be out of your sleeping bag while at camp?
I am going to be in the High Peaks for 4 days and 3 nights in about two weeks and am opting not to take any insulated pants... BUT, I might do so if traveling with a group. Groups (in my experience) tend to spend more time at rest and camp than I personally choose during winter outings. Thus, there is a greater likelihood for getting chilled. As John said, I would contact your program and ask for their recommendation as well as an itinerary for the trip- how long are the anticipated rest stops during the day?
My routine: I almost never stop for more than 10 minutes during the day- when I do, I wear my belay parka. If my schedule included cooked lunches or longer stops, insulated legwear would be welcome.
Once in camp, the process of stomping a snow platform, digging a trench, and setting up my floorless shelter creates a great deal of internal heat- I do not even wear my belay parka while setting up camp. Once set up for the night, I get inside and make dinner/water with my legs in my sleeping bag and wear my belay parka. Thus, I do not need insulated legwear. If I were to hang out with a group or stroll around an area doing light chores after dark, I might want them. However, your fleece pants should suffice.
Being that I would only use them in camp, I would not see the need for side zippers (if the day included long rest stops, I would). Thus, if you want a cheap pair of completely unfashionable insulated pants, these are an option:
Mine weigh 10 ounces and are a bit warmer than 200 weight fleece- they might be lighter and/or warmer than your fleece pants. They reach about to midshin.