salary.com is only one benchmark. Also look at glassdoor.com and what they've told you. Also look at the website they have that tells you what senior leaders are paid at your non-profit, that should be disclosed somewhere, I know there's a website. Gives you a sense of how much higher/lower you might be paid.
Also base pay is only one source of benefits, look at time off, commute options/work from home schedules, 403b match, number of direct reports, benefits, anything else you can use as leverage if they don't compare to your current role.
when salary came up before, what EXACTLY did you say, and what was the response?
when talking about salary, try to be the last to speak and to not let them know exactly what you make now (both of these are hard, I know. Also, if you are talking about ranges, start talking on the high end and work your way down. For example if you want 70k, start talking about 70-75k, not 65-70k.
Finally, whatever the offer, ask for 24-48 hours to think about the offer while you don't say anything stupid. Don't just take whatever they give you, esp. not the first offer.