Short version - it sounds like the Portland chapter is using their walks as fund raisers.
Long version - as far as I could tell, the local chapter in Columbia SC wasn't very active before COVID came along. But in the last three or four months they've put together three walks, and have coordinated with other agencies for a couple of conservation activities. At least two of the walks were free; I don't remember about the third.
The Aiken SC-Augusta GA chapter is an hour down the road. They do walks at least monthly, often more frequently. The only fees they charge are to pass along what's charged by the facility their visiting.
Pre-COVID, the regional Carolina Bird Club held quarterly meetings, with each quarter held in a different part of the state. Their programs offered a three-day weekend with over 20 different full- and half-day walks for you to pick and choose. The club's registration fee was a flat $40 regardless of how many you picked, plus any fees charged by the individual facilities for the walk you selected. They're trying gear back up again in January at the Outer Banks.
Are there group opportunities in the area other than Audubon? There's a city park here that has a volunteer-led walk monthly. The park admission is $5 but the walk is free. A local garden center also guides walks periodically for around $15.
So yeah, $65 is not just frickin' ridiculous, it's exclusionary. I'm interested in knowing what Portland spends the money on.