I think the price is still good at 20K. As far as lower numbers or letters on anything - they don't really affect the value unless it was owned by someone connected to the book in some way - the author's or artist copies, a special industry connection, etc. Other than that, if the limitation increased on a series and you can only obtain a full set under a certain number like the Dark Tower series - anything under #500 is worth more.
There are several companies that specialize in collectible insurance such as American Collectors or Collect Insure. They might be cheaper than a rider on your homeowner's. You can get quotes online pretty quickly.
They will look at market value. Just take the highest listings or sales you can find and they usually accept that if you have a claim - you decide what you want to insure for. Use different sources like Camelot, Ebay, Abebooks, etc.