AITA for refusing to attend my friend’s wedding due to a dress code?

My friend, who is getting married in a few weeks, sent out invitations with a specific dress code - black tie with black suit for men and floor-length purple or rose gown for women. I don't own any floor-length gowns. I asked the bride if it would be okay to wear a knee-length dress with dressy shoes, but she insisted that I follow the dress code or not attend the wedding at all.

A little back story, she introduced this guy she's been dating to our friend group. She told us they would be getting married in 6 months and we were happy for her. She specifically told us about the dress code she had in mind which we all agreed to.

But the issue is, she sent us a wedding invite after about a month of this meeting that she's getting married in two weeks. Her boyfriend is traveling abroad for a year for work so they decided to have the wedding before he left.

I was in no way prepared for this financially because the dress our friend group suggested was a bit pricey. I only agreed to it initially because 6 months was enough to save up for it.

I decided that I didn't want to spend a lot of money that I don't have, on a dress I would never wear again, and so I politely declined the invitation. I told my friend that I wished her all the best on her special day, but I couldn't attend due to the dress code.

She got angry and accused me of being selfish and unsupportive. She said that I should have made an effort to follow her dress code, as it was important to her to have a certain aesthetic for her wedding. And that, she informed me earlier so she feels I should have made preparations towards it. She then said I could rent one. When I checked the rental price, it wasn't necessarily cheap either. I would need my whole month's pay to rent the dress.

I don’t understand why not attending is better than attending without the dress code.

Now, I feel guilty for declining the invitation, and some of our mutual friends have also expressed disappointment in my decision. I understand that weddings are important events and it's important to show support for your friends, but I also feel like it's unreasonable to expect everyone to buy expensive clothes just for one occasion, and in my case, on such short notice. It's one week to the wedding and I feel terrible. Can I make it right?

AITA for refusing to attend my friend's wedding due to the dress code?