OK, I chatted with Verizon and found that if I have my ipad on its own contract, it's $30/month, which is the extra I'm paying now to have it on the family plan (I'm not paying an extra $10 to add the device, so I was wrong in my OP). I had assumed that there would be some sort of base fee if I went solo.
I suppose $30/month for 2 GB isn't the end of the world. I've never gone over that, as I don't stream videos or listen to music when I'm out of wifi range. It's strictly email, sometimes GPS, and sometimes internet browsing.
I am trying to figure out what to do with the other 5 devices on our plan, though...(that's another thread).