It's cheaper at equivalent level of best practice:
* For a clean architecture you want to isolate each Mongo and node process in its own system. So you need 6 instances, not 3.
* You'll need load-balancers in front of these node instances. That costs extra on AWS, and is included on dotCloud.
* Did you include the cost of bandwidth and disk IO in your estimate? Those are extra on AWS, but included on dotCloud.
* Monitoring is extra on AWS. It's included on dotCloud.
* I love to have a sandbox version of my entire stack, with the exact same setup but separate from production. That's an extra 2 instances on AWS (+io +bandwidth +load-balancing +monitoring). It's free on dotCloud, and I can create unlimited numbers of sandboxes which is killer for team development: 1 sandbox per developer!
* We only charge for ram usable by your application and database. AWS charges for server memory - including the overhead of the system and the various daemons you'll need to run.
* For small apps specifically, you can allocate memory in much smaller increments on dotCloud, which means you can start at a lower price-point: the smallest increment is 32MB.
I didn't even get into the real value-add of dotCloud: all the work you won't have to do, including security upgrades, centralized log collection, waking up at 4am to check on broken EBS volumes, dealing with AWS support (which is truly the most horrible support in the World, and we pay them a lot of money).
+ Our support team is awesome and might even fix a bug in your own code if you're lucky :)
Recommendation is to validate "best practice" claims. Doesn't matter what hosting solution used. Measure, measure, measure to make sure not only are you getting said claim but also that the end result meets your expectations. An example is in the past I had 7 "instances" (as shykes points out make sure they are hosted on separate nodes!) 4 of which where load balances Python web app. One of the nodes was overloaded so 1 out of 4 requests was very slow (5-10x). This was a big ajax app so initial page load would hang on the request(s) to that one instance. My point was since I had measured I could see that the node was the problem and now that I am on dedicated EC2 each node is consistant. Good luck.
That's good advice. As the saying goes: "trust, but verify".
Regarding your performance issue - most platforms (including dotCloud) enforce ram and cpu separation between nodes, but are vulnerable to IO contention at some level. This is also true for EC2 if you use EBS: your standalone instances will almost certainly, at some point, suffer from degraded and inconsistent performance because another instance is competing for IOPS [1].
You can avoid this with the new "provisioned IOPS" volumes [2], or by skipping EBS altogether for stateless components.
3 AWS Small instances cost under $200 / mo and come with 1.7GB of RAM each.
The dotCloud pricing calculator is coming up with $700 / mo for 3 mongodb instances with 1.7GB of RAM.
Obviously this isn't an apples to apples comparison. But How are dotCloud instances different from AWS instances?