i do not believe you can get the report you are after. If you run the special price for the whole day, then you can simply keep tabs on that day...but most likely it's on special for only part of the day.
To set this up for better tracking you would have to create separate items for the beer at the lower price. (you could use a transformere for this to make it easy to order)
if you keep the special beers in there own numbered range...say between 9001-9099 then you could set up a contest for this range of PLU's.
A detailed sales report is only going to show you how many or how much (in dollars) you sold of something. The dollar value is taking into account the special pricing...(some at $5, others at $2 on some days...)
Hope this helps....