According to a report by computer security researcher Brian Krebs, the price to purchase 1,000 registered Yahoo email address is $10, or 10 cents per account. The price to buy 1,000 Hotmail email accounts is $12, and the price to buy 1,000 Gmail accounts is $200, or $5 per account.
The reason that Gmail address costs more than Yahoo and Hotmail is due to the additional phone verification steps that Google requires in order to register an email address.
These bulk email addresses are used for various means, from spamming to registering spam Twitter accounts.
Source: Brian Krebs, “Buying Battles in the War on Twitter Spam,” Krebs on Security, August 14, 2013.