As far as dollars go, you pay $50 plus tax for the annual
DVC golf card which entitles you to the discount for a year -- actually the card lasts more than a year as it runs to the end of the month one year from the month you purchase it in. For example buy the card on January 10, 2007, and it is good until January 31, 2008. You can make tee times before you actually buy the card, which you purchase at any of the pro shops the first time you show up to play. For peak season you can make tee times 14 days in advance and for others 90.
The smallest discount rate dollar-wise is that summer season rate. You can play for $35 a person after 10 but during the summer season (May to September) the normal after 10 rates (called after 10 "price-slice" rates) run from $45 to $65 depending on course (Palm and LBV are cheapest, Osprey and Magnolia the most expensive; Eagle Pines, as I understand it, is now closed because of the Four Seasons construction). Note, the normal rates for any season can also change (and in the past they have gone both up and down sometimes).
During Oct to about mid-January (the actual dates do change somewhat every year and peak rate starts Jan 18 in 2008), that $50 per person per round charge compares to regular rates of $119 to $135. During peak season that $89 charge compares to regular rates from $135 to $169.
With an AP, you get 30% off regular rates at any time regular rates apply (and you do not have to buy any additional card). Thus, it can be used for times before 10. However, it is not useable for times when other discounts already apply -- for example, during the summer, it is not useable after 10 because regular rates are already reduced to the "price-slice" discount rates and it also cannot be used during lower cost twilight times.
Also a note to MinnieGi who is apparently going sometime in fall when $50 DVC discount rate is in effect. You note that last time play was at 1:45. There are twilight rates that begin at 3 before end of Oct and at 2 between end of October and mid-Jan and those are between $60 and $65 a round. In other words before deciding to spend $50 on a DVC card, you should consider time of day when you will play and whether you instead just want to go with times during the twilight rates.