When we first joined DVC in 1995, we paid cash for weekends on the advice of our DVC sales rep (taking advantage of the 25% discount off of rack rate). This way we could stretch our vacation from 6 days on points to 9 days with a combination of points and cash. We had 210 points to spend, and a 2BR at OKW was 30 points a night during the week. So we did 5 days on points, 2 days on cash, then 2 more days on points.
Since we purchased a 230 point resale in 1999, we just use points for everything. I divide the total points needed by the number of days we stay, and I came to the same conclusion as Richyams.
Least expensive: Stay 5 nights or less, avoiding weekends
Next best: Stay 6 nights (1 weekend) or 12 nights (2 weekend)
Everything else causes the average points needed per day to increase.
We just happen to have (almost) enough points to stay 12 nights a year at OKW in a 2br, so our cash paying, room changing days are over.