from a taiwanese guidebook titled 無料北海道 (literally, free hokkaido), found the following onsen place at otaru, with free bus service to and from sapporo.
the onsen entrance fee is ￥600. the train fare from sapporo to otaru is ￥620.
taking the onsen bus would let you soak in onsen for free, and save ￥20!
who says japan is expensive?..