Sometimes there are lots of tweens at the BUN park days and sometimes there are none; sometimes where there are lots of other tween boys, they ignore my 9 year old, since he is new and rather soft spoken or introverted. If you are on the email list, you can post a note saying you will be there and ask whether there are any girls your dd's age who will be coming. I think the HomeGrown Kids park days are similar but haven't been to more than one.
As far as the issue of most homeschoolers being fairly well off, I know what you mean and it's hard. We aren't really working class but my husband is the son of a plumber and tends to be on the low end of the high-tech pay scale which with one income is still pretty limited.
There is a mom in the SFBUN, Cristal Banagan, who has started her own group, called the Daring Kids Club, that may meet your needs.
Here is the link:
She has also started a consulting company:
She lives in Richmond.