Attendance to a workshop
Edit: Just noticed this was 3 years old. Hope the answer is useful to others.

I had a similar issue (though I finished my undergraduate at the same university(UK) I was given my PhD position.)

I had submitted a poster and was accepted at a conference. However, the conference was in September a month prior to my start as a PhD student and 2-3 months after I ceased to be a student. Technically as I was no longer a student at the university they had no responsibility to pay for me, but never forget if you are even a future PhD student they want you to do well. 

Together with my future supervisor we contacted the person in charge of the budget for conferences and explained the situation, along with all the relevant costings for attendance. 

It was accepted.

My answer to you is:


First you need to do your second question. In my field of Computer Science the websites offer "early bird" attendance rates, rates for students, and in some cases of large conferences offers on payment of accommodation etc if you fall into certain categories (for example if you want to volunteer to help). Once you know the true cost you can do the second point.
With your supervisor(they will know who is who) contact the budget holder for conference attendance at the University you are going to do your PhD at. As above, state what is going on, why it is important and how it will help with your future as a PhD student at that University.


Hope you get your workshop approved!