I agree you should push hard with the store manager, but at the end of the day, it doesn't sound like it was their fault. They are basically offering a "goodwill" discount to you. I have a hard time saying they should have guard rails and that sort of thing. One other option you could consider is having the glass fixed by one of the many vendors such as iresq.com. This may be the cheapest option, but I agree with the person who said there could also be some internal damage.
If you do get a new iPad consider getting the insurance. AppleCare+ has become much more competitive and is less restrictive than it used to be. Square Trade is also a safe option. If AppleCare can't fix your device, they will give you a refurbished one right in the store. If Square Trade's repair vendor can't fix it, they give you the replacement price, which is great. My wife dropped her iPhone on the driveway and shattered the glass. Square Trade sent us a box to mail it in with (they paid both ways), and they quickly sent it back with perfect new glass. However -- there was internal damage to the antenna and it couldn't hold a 3G connection. We sent it back again, they acknowledged that it was not repairable and quickly gave us the "full replacement" cost which was around $500 vs. the $200 we paid because we were on contract. AT&T still gave us a contract discount, so we actually recovered more than we needed to replace the device. Unfortunately, you can't turn back time and get the insurance on the broken device now.
Good luck.
