This may be better explained through a specific, though hypothetical, example. Even in this day and age, the going rate for trees can't possibly be that much. Perhaps part of the issue is the procedure for determining class gift options. In an effort to come up with an idea that will actually be carried through, classes may cunsult with the administration as to minor projects that are already approved. This method is helpful to the college in that the little things add up, and any contribution to the ongoing facelift is greatly appreciated by the administration.
Memories are the result of impact.
A gift that impacts a population beyond the superficial has a greater chance of being appreciated and remembered. Perhaps the idea of actually making a difference would increase the number of seniors willing to pledge, in turn providing funds for truly efficient improvements to the campus. It's perfectly understandable if funds aren't available to complete a project that will directly affect future students. However it is one thing to provide for a current administration project to relieve a burden and quite another to throw money towards plaque-adorned cosmetics. Prospective benefactors must keep in mind that giving and receiving are opposites, and while one may lead to the other, the reciprocation of a gift may not come in tangible form.