A podcast that I listen to had discount rates when ads hadn’t been filled. I took out an ad talking about how I had no product or service to sell. It was a hilariously funny private joke for me and the guy who approved / read the ad. I doubt anyone else enjoyed it as much, but at the discount rate, it was completely worth it.
I think maybe I paid $150 at the time. The podcast was Grumpy Old Geeks. I'm not aware of them running any discounts now, but you never know if they'd cut you a break if you reach out. Podcast advertising has apparently been pretty dried up since the pan.