Starting with SQL Server 2014, Developer edition became free for Visual Studio Dev Essential members which is free to join. If you are not a member, I highly recommend signing up by clicking here and taking advantage of some of the awesome perks including free training, Azure credits, developer tools, etc.
Developer edition is essentially the same thing as Enterprise edition (just with a different name) and should only ever be used on non-prod systems.
Prior to this, you either had to have enough MSDN\Visual Subscription licenses for non-prod to cover those accessing it or you had to pay ~$50 per user. If you are running older versions in non-prod and are not covered under a subscription model, then I would recommend speaking to your licensing rep about that to see if you need any additional purchases.
One of the biggest issues I have had with Developer edition is that it does not reflect my production server build if I am running Standard edition in production. The last thing you want is to have something developed or tested using a feature set that is not going to be available once it gets to production. My understanding is that you can run Standard or Enterprise (though why would you since Developer is the same) in non-prod, however, you must be covered under MSDN\Visual Studio subscriptions to do this. Or another option is there are several articles posted online about how to make Developer edition installs function like Standard edition though have never tried this. A quick google search will lead you down this path if interested. The good news is that there are more and more features being added to Standard edition in later versions so the issues with Developer edition vs Standard may not be an issue.
If you have any questions or concerns about whether you are covered, I would highly suggest posing any questions to your Microsoft or licensing rep.