There is not a nightly fee.
Your $20,000 is buying you a number of points, which is the unit of currency for DVC units.
For example, a 1 bedroom at Wilderness Lodge Villas for a week during the least expensive season will "cost" 200 points. That same room during Christmas week will cost 350 points. (Sun - Thu is fewer points per night that Fri & Sat, and many people work the system to get maximum value from thier points. Since you're on the other side of the earth, you're unlikely to fly to Florida for 4 days, and I am assuming you will stay at least a week.)
There are additional yearly charges after you pay your buy-in cost of $20,000 (or whatever). You are assessed around $4 per point each year.
That $89 amount sounds like the going rate for a point these days.
It does seem confusing, but hang in there. There are lots of seasoned veterans around here.