Way back when, I had an economics class which talked about baking bread and pizzas in the same oven, (different temps, so not at the same time of course) so if you're spending time doing X, you can't use that same time/space for doing Y; therefore doing a "free" appraisal is costing your time at the very least when you could be doing something more productive, like doing an appraisal for PAY, or even taking the day off and having a snooze, which could also be beneficial to you. So FREE ain't FREE. Also, what are the odds of the person who is volunteering your time being willing to give up THEIR full day's gross income, and then having to re-work their monthly budget, let alone having the potential of the recipient coming back to waste more of their time to explain why this or that value/adjustment and potentially argue about it?
Yep, a $50 donation is a better bargain, and make sure your name and company are emblazoned on the donation card so they remember you donated to their cause.