When declining the invitation to be a bridesmaid, it's important to be honest yet tactful. Here's a polite way to express your situation:
"Thank you so much for considering me as a bridesmaid for your special day. It means a lot to me that you thought of me in this role. However, I must be honest and share that I am currently facing some financial constraints, and the cost of being a bridesmaid, along with the time commitment, would be quite challenging for me at this moment. I wouldn't want to take on the responsibility knowing that I may not be able to give it my fullest attention and support.
Please know that I'm genuinely honored by your invitation, and I'll be there to celebrate and support you in any other way I can during this joyful occasion. If there's anything else I can do to contribute to your wedding preparations, I would be more than happy to help.
Wishing you all the best on your big day, and I hope it's filled with love and happiness."
This response conveys your appreciation for the invitation while also being upfront about your limitations in a respectful manner. It shows that you genuinely care for the bride's happiness but are currently unable to fulfill the obligations of being a bridesmaid.