When you're shopping for a paid product, you're generally trying to minimize your costs (while balancing quality). When you're donating to a free product, you're actually trying to maximize the effectiveness of your donation. If you were simply trying to minimize your cost/benefit ratio, you would donate nothing. Clearly there is a totally different mentality at play.

Consider it also from the recipient's perspective. Their benefactors are more likely to donate more money when they believe it will be put to good use. It's a complicated messaging problem, but being vague is probably not in your best interest.