WWDC stands for Worldwide Developer's Conference, Apple's annual developer event. There they announce new versions of iOS, MacOS, SDKs, and developer tools, followed by four days of talks, events, and labs. The talks and labs cover every aspect of app development, focused mostly on writing and debugging code. The talks aren't just what's new; they also present deeper explorations of technology that has existed for years.
Outside of the presentation halls, there are numerous places where developers can mingle and collaborate with each other. There are rows of tables with power and ethernet where developers write code and discuss what they've learned. There are outdoor terraces where developers can sit and chat. There are also industry-specific meetups, like health and games, where developers and Apple employees can share there knowledge.
WWDC has an extensive set of labs staffed by Apple engineers (and a few experts from areas like design and marketing). In my experience, these labs are the most valuable part of the conference. The labs area is divided into twelve different segments. Apple engineers with different technical expertise populate each area for an hour or two before another topic takes over that area. There are Apple experts in topics like AirPlay, Privacy, Swift, Marketing, Internationalization, Bluetooth, Metal, Accessories, and so on.
WWDC costs USD $1599 to attend. If you need a hotel, expect to pay $300-$500 per night. San Jose isn't a cheap town, and the more affordable hotels are typically far away or falling apart. The hours of WWDC are long, typically from 9am to 8pm. I recommend booking a hotel within walking distance so your developers aren't trying to battle sleep deprivation.
I typically budget $3000 - $5000 for WWDC per developer.
To put this in perspective, the total cost of WWDC is about the price of the nicely configured MacBook Pro your developers use. It's less than two weeks of a US-based iOS developer's paycheck. It's a lot less than the recruiting fee for an iOS developer, which might total more than $30,000.
When I first started developing iOS apps, buying WWDC tickets was easy! You just bought a ticket.
These days, the demand for WWDC tickets far exceeds the supply. Apple distributes tickets on a lottery basis. Your developers need to register for the lottery and put in their credit card information. If they win a ticket, their credit card is immediately charged the full cost of WWDC.
Apple does not offer refunds, and will only allow you to exchange tickets within your own organization (and I believe you must ask Apple for permission).
For this reason it's critical that you inform your developers about your WWDC policy as soon as possible. Your developers are putting their credit card on the line for $1599. You'll really upset your developers if you tell them your company won't pay for WWDC after they paid for a ticket!