Part of the info you're looking for would be in their financial statements in an aggregate basis. The mix of who contributes what varies from appeal to appeal. Could be partner orgs, board members/ABT officers/in kind from property owners.
Accountability The audited financial statements and their Charity Navigator
page may help address some of your concerns.
It costs them around 6 cents to raise each $1.00 per Charity Nav.
I appreciate you taking an interest in preservation and being willing to step up. I've been an ABT member for a long time now and donate regularly.
Can definitely second this, they've been very effective in preserving land and items relating to the fighting in the Valley throughout the war. Their
Hoke's Run and
Manassas Gap Railroad appeals may be of particular interest to CWdiaryPA for the connection to Gen. Jackson or passes through places his troops fought. My membership with SVBF is of more recent vintage but no less enthusiastic.