We're designing a system for a client that involves quite a bit of data. The regular data storage limits in Salesforce will definitely be exceeded by a factor of 10 or more. So one technology that seems like it might be cost effective is to store large static sections of data remotely and access it using External Objects/OData/Lightning Connector. Is there a way that the connector can be turned on in a sandbox or dev org so that functionality can be tested and verified before purchasing the Lightning connector?
Developer Edition orgs provide a single OData External Data Source. It has lower limits than a production data source (1,000 OData queries per hour rather than 10,000 - see docs for more), but works just fine for proof-of-concept.
Good luck getting an account executive to turn on the OData for an existing developer org if you don't have it. I've checked two of my existing orgs, one from 2007 and another from 2012, and they both have this feature enabled. One has three Lightning Connects (Salesforce, OAuth2.0, and Simple URL), while another only has two (OAuth2.0 and Simple URL). I would think that you should at least have access to Simple URL and/or OAuth2.0 available for testing in your Developer Org. If not, you should be able to create a new org and have it available (all new orgs should get it, from what I can tell).