With glass, doesn't the glass dull a blade fast?
I have been using glass for as long as I have been making signs so I can't say if glass will dull the blades faster than a cutting mat. However, I know from talking with other sign makers that a cutting mat, even those that claim to be self healing, will eventually hold a scar and can make your blade wander.
Honestly, X-Acto blades are about $3 / thousand. I know that is an exaggeration but are you really worried about X-Acto blades costs? I would much rather change a blade then re-cut, re-weed and re-mask something I just screwed up because my blade caught a scar and cut into my vinyl.
BTW, if you go with glass make sure it is at least 1/4" thick and tempered. The thickness will vary depending on how big a table you are covering. Also, put some of that non-skid rubber foam material between the table top and glass (the stuff they sell for under an area rug). It will keep the glass from sliding and also absorb tiny bumps which might otherwise cause a stress point on the glass and crack it.