I'm sure others have referrals but it's tough to find something in Chatham for that price. You can either go north a bit, or back toward the mainland to get into that range. Mid-cape, there are a lot of low-cost rooms along rte 28 in Yarmouth for example, just east of Hyannis. This location makes access to Bass River, the south side beaches and bayside flats very easy, and the drive to almost any place on cape is within 1 hour.
Past the Orleans rotary, you'll find additional lodging under $100 heading north on rte 6... making outer beaches access a snap.
Any body have specifics? I'll grab a directory next weekend.