That confused me too - the https://postgres.heroku.com/pricing page should definitely mention that it's per month. I'm also not clear on the difference between the Ronin and Fugu plans, both of which are 1.7 GB of RAM but one which is twice the price of the other.
Ronin = Small Instance: 1.7 GB of memory, 1 EC2 Compute Unit (1 virtual core with 1 EC2 Compute Unit), 160 GB of local instance storage, 32-bit platform. ~$60/month spot.
Fugu = High-CPU Medium Instance: 1.7 GB of memory, 5 EC2 Compute Units (2 virtual cores with 2.5 EC2 Compute Units each), 350 GB of local instance storage, 32-bit platform. ~$120/month spot.
Ikea = Large Instance: 7.5 GB of memory, 4 EC2 Compute Units (2 virtual cores with 2 EC2 Compute Units each), 850 GB of local instance storage, 64-bit platform. ~$240 / month spot.
Zilla = High-Memory Extra Large Instance: 17.1 GB memory, 6.5 ECU (2 virtual cores with 3.25 EC2 Compute Units each), 420 GB of local instance storage, 64-bit platform. ~$360/month spot.
Baku = High-Memory Double Extra Large Instance: 34.2 GB of memory, 13 EC2 Compute Units (4 virtual cores with 3.25 EC2 Compute Units each), 850 GB of local instance storage, 64-bit platform. ~$720/month spot.
Mecha = High-Memory Quadruple Extra Large Instance: 68.4 GB of memory, 26 EC2 Compute Units (8 virtual cores with 3.25 EC2 Compute Units each), 1690 GB of local instance storage, 64-bit platform. ~$1440/month spot.
I think Heroku will pay less than the spot price for the raw server, but fork out bandwidth, storage, EBS IO, backups and so on. Amazon sells its basic servers cheap, but virtually nothing is included.
It looks like their margins go up at Zilla, but maybe the high-RAM instances need more bandwidth and backups.
