I'll make some assumptions... that you pay $80 per point. Finance for 10 years. And the interest rate is 10%. I really don't have any idea what the rate is now, but a 5% swing in either direction would make about a $10 per month difference.
The monthly payment for the above scenario would be around $53 plus your MF of $4.21 x 50 = $210 / 12 = $17. So bottom line is around $70 per month.
Earlier this year DVC was offering SSR and AKV incentives on add-ons as low as 50 points (Seemed to take advantage of the tax incentive)
dvcnews mentions current $8/point on 50-99 point AKV contracts and more incentives on 100+ points, so it may still exist..