I have the same question. I'm considering attending my first retreat at Spirit Rock sometime later this year. I am 100% aware that my approach to determining an appropriate amount of dana is not at all what is recommended, and my approach may even offend serious Buddhists. The guidance I have received is always along the lines of, "dana means 'virtue of generosity', and no one can give you guidance on your generosity or the value of the instruction you have received." However, that is just not how my brain works. I need to run some numbers to get a rough estimate to determine a range.
I'm looking at a few retreats of varying duration but most are about 5 days, there are usually 3-4 teachers listed, and dana covers administrative staff and the cooks in addition to the teachers. Based on some pictures I've seen it appears there are 40 students per retreat. Therefore I assume that there are about 6 full-time staff supporting the retreat, this should cover 1 week (5 days) of a reasonable salary, and it needs to be split among 40 students. If I decide that a reasonable salary for each person averages out to $40k/year, then that's $240k/50 weeks = $4,800 split among 40 students = $120/student. These are extremely rough numbers, but hopefully they get me into a rough ballpark range that's appropriate.
Note: I do no plan to just give $120 - this is just to help me figure out what a reasonable range might be. For someone with a modest income or modest assets my numbers suggest $120 could be an upper bound. I would think that for a 5 day retreat someone with more substantial economic means should consider that to be a lower limit, and only if the instruction really isn't very good.
My hope is that I'll get so much value out of the retreat that I will consider the value to far exceed $120 and I will want to give substantially more. If I spent 5 days on a meditation retreat and only felt that my life improved by $120 worth then I would consider it a failed endeavor. But at least by working through some rough numbers I suspect that for the retreats I plan to attend $15 is too low and thousands of $ is too high unless you have a life changing experience and you have substantial financial resources.