To my knowledge, there are no discounts on single day tickets. If you're going for 2 days, there's a way to have the 2nd day discounted (I think $5) when you purchase them at the same time from a theme park main gate. (For 2 days it is cheaper this way as opposed to the Play4Days pass)
Here's the info from Mousesavers(prices include tax):
One-Day/One-Park Ticket
This ticket is available ONLY at the park gates. You cannot purchase it in advance. It provides admission for one day, to one of the major parks: Magic Kingdom, Epcot, Disney-MGM Studios, or Animal Kingdom. No "hopping" from park to park on the same day is allowed with this ticket.
$55.38 Adults (10 and older); $44.73 Children (3-9)
Add a second day of One-Day/One-Park admission when buying the first day and save $5 plus tax on the additional day. The second day can be used at any park (does not have to be same park as first day) but you cannot "hop" between parks on the same day. Expires 4 days after purchase. Total price for two days' admission when purchased this way:
$105.44 Adults (10 and older); $82.01 Children (3-9)
Add a third day of One-Day/One-Park admission when buying the first and second day and save a total of $10 plus tax on the additional days. The second day can be used at any park (does not have to be same park as first day) but you cannot "hop" between parks on the same day. Expires 6 days after purchase. Total price for three days' admission when purchased this way:
$155.49 Adults (10 and older); $119.28 Children (3-9)
Please note: don't call Disney and ask about the 2 day and 3 day One-Day/One-Park admission. It's a waste of time. They will tell you it doesn't exist, because they don't sell it by phone. It is available ONLY at the park gates!
