Sorry you're having trouble finding affordable lodging for your trip. As mentioned, Big Sur is VERY popular, but there is more demand than supply for lodgings, so rates are reflective of that fact.
While I understand you don't like the idea of driving into Big Sur from Monterey or Cambria, you will find more affordable options in those towns in comparison. The drive into the "village" area of Big Sur from Monterey is a little less than an hour, while it's close to a 2 hour drive from Cambria. That pretty much makes Monterey a more viable option.
You can overnight in Cambria or San Simeon before embarking on the drive up through Big Sur to Carmel/Monterey or book one of the available cabins at Ripplewood or Fernwood.
There's a VERY affordable Motel 6 in San Simeon that should suffice for an overnight stop. It used to be a Holiday Inn, so the rooms are a little larger and nicer than your typical Motel 6.
Lodging options in Cambria include Bluebird Inn, Creekside Inn and Cambria Palms.
Don't discount the Monterey Peninsula, either. It offers some stunning scenery via Point Lobos, Carmel Beach, Asilomar Beach and the 17 Mile Drive.
Monterey lodging: Casa Munras, Borg's Oceanfront Motel (Dated, but clean with million dollar views of the Monterey Bay from their street front rooms), Casa Verde Inn, Comfort Inn on Munras, Quality Inn on Munras.
There are also a few B&B's that include breakfast and evening repast that can help defray the costs of eating out: Centrella Inn, Gosby House Inn, both in lovely Pacific Grove, which borders on Monterey and Carmel.