I typically buy at least two titles per month, and the best deal ended up being:
Audible Premium Plus - 1 Credit Every Other Month for $17 ($8.50/mo)
You can buy 3 more credits for $37.99 (12.66/ea). It’s also worth checking individual titles because quite a few cost less than the credits.
Correction: I guess I actually buy slightly fewer than 2/mo because there’s a plan for that ($22.95/mo) that’s cheaper than the 27.50/mo from my numbers above. I had that one for a while but ended up feeling pressured to use them before they expire.
You probably won't be interested since it's even more pressure to use them before they expire, but there's also annual plans which are even cheaper if you can be happy using 12 (or 24) tokens within 12 months (you get them at the start and they expire at the end of the year):
Audible Premium Plus Annual - 12 Credits
$149.50/year (way cheaper in UK: £69.99/year)
Audible Premium Plus Annual - 24 Credits
$229.50/year (£109.99/year)
Although, as soon as I'm logged in with my account (UK) which had subscribed in the past but isn't currently, it doesn't seem to be giving me any options except to start a 1 month free trial for 1 token/month, not sure if other options aren't available or just extremely well hidden...
edit: no it is available for my account, though I'm going to remain a non-subscriber and keep using my local digital library :)
Audible Premium Plus - 1 Credit Every Other Month for $17 ($8.50/mo)
You can buy 3 more credits for $37.99 (12.66/ea). It’s also worth checking individual titles because quite a few cost less than the credits.
Correction: I guess I actually buy slightly fewer than 2/mo because there’s a plan for that ($22.95/mo) that’s cheaper than the 27.50/mo from my numbers above. I had that one for a while but ended up feeling pressured to use them before they expire.