Free training at Bay-Area Drupal Camp is time well spent for Drupal developers.
Bay-Area Drupal Camp (BADCamp) 2015 was a great way for users of all levels to learn about Drupal. Hosted by UC Berkeley, all events were free. The first two days were dedicated to all-day training sessions. Below are two examples of the several training sessions offered to developers and themers.
Lecture and hands-on exercises by Four Kitchens
Drupal is a full-featured CMS, and site owners are not eager to give it up. However, by itself, a Drupal site's functionality is limited. You might want to try newer web technologies with your existing content; you might be wrestling with Drupal's presentation layer; you might have many different sites or consumers for your data. The Headless Drupal training session covered how and why to make your Drupal site headless; that is, using your Drupal site to serve data with a REST API and not serve as a destination for human visitors.
The main issue to overcome is how slow Drupal can be and how to work around cached content to keep your API data fresh. The benefits to this approach are two-fold: development can be faster by separating work among functional teams (one team to work on creating an API and another team to work on front-end development) and content is available to other non-Drupal sites.
The solution created by Four Kitchens relies on the Restful module to serve data. They prefer this module because it uses object-oriented architecture for creating custom endpoints, enforces API versioning, and includes test coverage.
You can read more detailed information at http://www.mirzu.com/headless-training-book/.
Online training and in-person mentoring by various experts
BuildAModule is an online video training series. At BADCamp, the BuildAModule team decided to run the training sessions flip-the-classroom style, where attendees watch videos at their own pace and ask for help as needed.
At first, it didn't seem to be the best use of conference time; but, this was actually an excellent way to get answers from experts to specific questions. There was a team of friendly mentors available to answer questions about theming, coding, maintenance and hosting. And because the training took place in the conference midway, vendors were also available to help out with technical questions. Registered attendees were given a week of free access to the videos leading up to BADCamp, so you could come in with questions.
Priced at $23/month, BuildAModule is a great deal for every level of Drupal user and includes detailed, transcripted videos categorized by your use of Drupal (themer, developer and others).