This is going to be difficult to breach, even in a company that has money to spare, because all business expenses will be treated with scrutiny when you bring them to the manager.
You need to prepare yourself with as much knowledge about the workshop as you can. Correlate as much of your on-the-job duties as possible to the training that this workshop offers, highlight your own level of skill and the skill that this workshop promises to deliver to you for taking it, and if possible highlight how much added value to your workflow taking this course will bring.
You can, if you're very eager to take this workshop, offer to cover some of the cost yourself, depending on how important it is for your personal growth in your career.
Remember not to be intimidated by an initial opposition - if your group really is on a tight budget, you will have to fight a little to get this approved. Stand your facts up, and back them up as well as you can.
Also, make sure you present them in the best way possible to your boss - some bosses prefer face-to-face discussions, others prefer email. So cater your request to the way your boss prefers to be approached.