I actually just happened to ask my step-mom this the other day.
She said that last year SACC was around $400/month. It's hard to tell what it will be by the time we need it.
Here are the Ffx Cty fees for the school program: http://www.fairfaxcounty.gov/ofc/SACCSchoolFee.htm
I imagine you could find "in-home" before and after school care in your neighborhood. I know of at least one lady that does it in our neighborhood, though she charges an arm and a leg because she also tutors.
I live in Loudoun County and the kids can arrive at school at 7:30 so a lot of places (unless it is a day care) don't do the before school care. Where my son goes and CASA (the after school care) both charge around $275 a month. When my son was in Kind. we paid $475 for after K until we picked him up (11am-6pm).
Another big change from day care to after school is, most day cares will stay open until 6:30...the after school programs mostly close at 6. Those 30min take a little getting used to (esp around here with the traffic situation)