I'm not sure that charging for forum access would be a good idea. Jakob Nielson came up with the 90-9-1 / participation inequality rule. Basically, in an online community, 1% are active contributors while 90% are lurkers.

You have a product which is currently free - podcasts. Personally, I'd try to monetize podcasts, rather than the forum. If people are willing to pay 99 cents for a 4 minute song, would they also pay for a 60 minute podcast (and a community hub like wptavern)?

Perhaps people would be willing to pay 50 - 99 cents per podcast, or a 25 bucks a year subscription? I would.

Any significant news from the podcasts are included in the shownotes or a subsequent post, so it's not a case that the information wouldn't be 'out there' in the community. If people want to listen to the podcast itself, they should be willing to pay for the value you provide.

If it fails miserably, you could always pull a Brian Gardner and try something else :)