For most people (including every commercial project I've ever worked on), the time-saving and safety benefits of relational schemas was far greater than any theoretical infrastructure savings.
This is so dangerous even to type out :) In some, limited cases, Dynamo can replace tables in an RDS, and completely outperform it, too. I'm a big fan, but it's fundamentally different from an RDS, and you can get burned, badly.
Oversimplified, DynamoDB is a key-value store that supports somewhat complex values. If you have large values, use S3 instead  - I think that's a good way to think of it, a faster S3 for loads of small records (with a nicer interface for partially reading and updating those records).
If you need to look up on anything but the primary key, be careful, costs can get out of control by having to provision extra indicies. If your primary keys aren't basically random, you'll run into scaling issues because of they way DynamoDB partitions. If you need to look at all the data in the table, DynamoDB probably isn't the right technology (it can, but scans absolutely tank performance = $$$).
DynamoDB as a hugely scalable limited-scope data-source in your app are likely where you'll find the optimal cost/scalability point. By using Dynamo for scalable read-heavy activity your let the rest of the app be barebones and 'KISS', preserve competencies & legacy code, and retain the benefits of relational schemas. Dynamos scaling then becomes your cloud-cost tuning mechanism.
By way of example(s): if your editors all use RDS and you publish articles to DynamoDb you could be serving tens of millions of articles a day off a highly non-scalable CMS. If all your reporting functions pull from DynamoDB you could be serving a huge Enterprise post-merger while using the same payroll system as pre-merger. Shipping tracking posted/grabbed from Dynamo, purchasing logic on the best Perl code you could by in 1999 ;)
If you are going to do joins and that sort of things, forget about dynamodb. It cannot do that and for a good reason. That being said, our architecture is mostly SPA so the lack of joins is solved at the client - there are just more calls to services client-side but the affect of that is still cheaper and faster product to run and maintain.