Here's the scoop:

The BizTalk Server Developer Edition is coming and is scheduled to arrive on November 1, 2013. It won’t be free but the cost should be under $50 USD. In the meantime, these are your options: 

  • MSDN Subscription includes the Enterprise Edition. The Enterprise Edition and the Developer Editions are the same software package except for licensing. Meaning, the Developer edition cannot be used in a Production environment. It is for development use only.
  • Evaluation Edition is a 120-day time-bombed Enterprise Edition.
  • The Enterprise Edition on Windows Azure IaaS, aka BizTalk Server 2013 on a Windows Azure VM - If you have MSDN, you get $50, $100, or $150 in free Windows Azure usage per month. With MSDN discounts, a medium BizTalk Enterprise instance is less than $50 a month.

I hope this helps!