Hi Michael,
Dave Carroll here from salesforce.com. This is a great question and, as with anything related to enterprise app development, the answer is depends. I'll try to shed some light of what depends on what.
API access for the salesforce.com customer is dependent on the license that they use. We have several editions and only the enterprise and hight editions come with API access for standard users. As we realize that this limits the potential audience, formal partners can be issued a special partner ID that can be included in the API calls (in a header) to allow customers with a lesser edition to exercise the API. Basically we see that and say, oh, this is a partner exercising the API on behalf of the customer.
Another thing to think about is marketing the App. While I assume you can get an Appery.io app into Apple's App Store, that is not typically where salesforce.com customers go to find solutions. Rather they head to our AppExchange site to find salesforce.com specific apps. To play in the AppExchange, and thereby gain better visibility in our customer base, you need to have a formal partnership with salesforce.com, and that comes with a price.
Enterprise software customers have higher standards as to trusted applications installed in their salesforce.com accounts or apps that access their business critical data. The AppExchange listing process includes steps that all customers to trust those apps.
There is no direct cost to you as the developer for creating an app that uses a connected app, but you will need to create a package for the customer to install, again at no cost. If your app does not depend on any custom schema or server side code, you just need to install the connected app from your dev org into their org via package. The package is the main asset access when acquiring an app from the AppExchange, but can also be what we call an unlisted package. This means you would need to send a URL to the package installer to your customers.
There is some complexity when creating an App for salesforce.com customers, but the developer cost can be zero or be what ever the partnering costs are.
It depends!
Cheers