I use window or picture frame glass. The only issue you need to watch for is cracking due to uneven heating. Heat it up slowly and evenly and do not place cold items on hot glass. With that said, I run standard hot beds under the glass at full power.
The glass on my Delta is 1/4 thick window glass. This was done by the glass place next door. It cost about $30 with the polished edge. I have it heated by a 160 watt aluminum plate. I have run this as high as about 120C in about 8 minutes.
This is a pic from #ORDCamp. There was a robot drink maker in the next room, so I had it make my robot a drink.