I agree with everyone else in that 9 days is very short notice. If I were to receive a black tie invitation in that short amount of time, I would be unable to attend.
If your husband is urging you to go with him and this is relating to his career, I would probably attend -- but only if it truly could boost his career. I have attended many business parties for my husband and I do believe they can pay off in the long run. It's all about who you know. I haaaaaaate attending these parties! I don't like being out late, I don't know anyone there, and no one is ever my age (I am 28, hubby is 32). But, my husband is very good at getting to know the "right people" and it has really made a difference for him.
If the main reason is that you just cannot afford to spend the money for the proper attire, you can either consider renting a gown or decide not to go alltogether. I agree it is a lot of money for one night that isn't even fun, plus as you said you are already very busy around the time of the party. Either decision you make will be fine, just make sure and discuss it in more detail with your husband so you are both on the same page.