However, the rate-limiting factor might not be the cost, but the availability of unique credit card details.
Availability of cards isn't going to be an issue.
THere was a period where spammers would run up high charges on prepaid cards and (of course) not actually pay for the ads.
Costs per card about $5 bucks, and will get cycled into a multi-merchant hit to extract the most value from each card. A $1 Twitter charge, $50 bucks in digital gift cards, a "maybe it will work" hit for hundreds in electronics, or clothes, etc.
If nothing else, Twitter just made itself a GREAT place to test stolen numbers, since a $1 charge isn't likely to raise any flags and get the card shut down.
Virtual debit cards however are interesting. My bank lets me set up as many of those as I like and I don't even have to use my real name or billing address with them.
And yeah, there's also the stolen credit card / debit card market. I really can't see this adding that much pain for these bot handlers. It might make it a bit easier to identify the patterns at least.
I think this will just end up moving the problem further down the line and end up with twitter accounts being bought / sold.