$$ how much is enough?

Yep.  You can't work (paid or unpaid) while on a visitor's visa.  I do think that volunteering should be okay, but apparently it's not.  I guess maybe their logic is that you might get offered a job and be tempted to stay...maybe. So Stacey, you might want to NOT mention volunteering whilst on your visitor's visa...if you're asked about it.

Thanks  guys!!!  I had no idea!  That would be awful!!!