KWT,
Once you get out of the SEC and Big12, plus about 1/2 of the ACC and Pac12, the attendance at D1 baseball games varies widely, with probably 50% of all D1's having attendance about the same as that ILVBB has listed for Trinity University. I just spent a few days on a trip with a group which included 3 former Ivy players. Two of them played at Columbia before the recent success. Their description of fan numbers at Ivy games was not much different than D3, and less when the weather was cold.
As one illustration on the D1 level, look at UC Irvine. One of the top 25 programs nationally, but they probably average 300-400 people to their weekend games and less during the week, and they have a beautiful stadium.