There are plenty of places to camp for free. The Cinnamon Creek forest rec site by Loose Lady and Marble Canyon Prov Park both offer free camping in winter. Near town, try pulling off onto the gravel road near the Seton Lake BC Hydro campsite (which is closed in winter but there's never really a problem if you park outside the gate). In the Bridge you can camp at the rec site below Terzaghi Dam, right by Night N Gale.
From past experience, the problem is not so much free camping but not being able to dry your gear. Wet ropes, gloves and boots will freeze overnight while car-camping. On a 3 or 4 day trip this can reduce fun significantly. Sure, you can bring it all in the pub with you and spend a few hours drinking while your gear dries out but that gets expensive too. Getting 4 or 6 people together in a kitchenette motel room seems like the cheapest option overall.