My company wants to hold its first ever social event over the weekend at someone's house. At first I accepted the invitation, although I wasn't at all into it, but then I learned that all the costs for this event will be split over all attendees. It's not like a 5$-10$ thing, this would be equivalent to one or two days' work for me. I'm also budgeting and taking care of my ill Mom, so every penny counts. I'm not a cheap person, but this expense is a lot for me. So, I declined the event.
I expect people (including my manager) will be asking me why I declined. I don't want to lie and say "I have another thing scheduled". I want to make it known that I don't like to be forced to attend and also pay for it. However, I would like to say that in a professional way. I love my job and do many things free of charge, just because I want my company to be a leader in our field.
How should I respond if asked about this? How can I professionally convey that I don't want to pay for social events?