Glassdoor.com has a decent salary estimator here. It allows you to account for geography and a few other variables. I couldn't find a way to limit it to nonprofit / ngo only, unfortunately.
Another resource that is nonprofit-specific is called the annual Fair Pay for Northern California Nonprofits. As you can tell from the title, it's specific to northern CA. But you can factor in our high cost of living to guestimate what it might be in other geographies. This report is discounted to organizations who participate / contribute data, and they have a sliding price scale based on your organization's budget size.
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