Use

Map[costperfish[#]&,{{50,4,2.5,2},{20,6,1,2.5}}]

Hi Brandon, you're using the map function with just 1 argument. that why it doesn't evaluate further. I would suggest you to invest some time on learning the Map and Apply function because they are very frequently used! What helped me was the introduction of Mr Shifrin http://www.mathprogramming-intro.org/ and the video of Mr Gaylord are also very nice http://www.wolfram.com/broadcast/s?sx=gaylord

My bad :) You can do it in a lot of ways..

costperfish[{pondcost_, nsides_, length_, depth_}] := N[ pondcost/numfish[nsides, length, depth]];
costperfishOriginal[pondcost_, nsides_, length_, depth_] :=  N[pondcost/numfish[nsides, length, depth]];

The costperfish has now a list as argument and that is exact what you want. Now it's easy to map it!

data = {{50, 4, 2.5, 2}, {20, 6, 1, 2.5}, {30, 5, 1, 2.5}, {20, 7, 1,   2.5}}

Map[costperfish[#] &, data]

or

Map[Apply[costperfishOriginal, #] &, data]

both give {1., 0.8, 1.76471, 0.555556} as a result