We've decided to try Thinkwell math next year, but have been dreading the cost for two kids. Homeschoolbuyersco-op.org will be running a special March 21 - 28th. You can add your name to the notification list by visiting http://homeschoolbuyersco-op.org/interest-meter/_id_12=Thinkwell Hope they do well with this, so Thinkwell will participate again.
Here's a site I found today while I was searching... it might be as good as Thinkwell, and I think it is cheaper. Either way though I thought you might want to take a look. http://www.tabletclass.com/