First you should decide on what type and thickness of glass you want? Clear? Frosted (Overall or with a custom design)? Tempered/Safety? Smoked? Multi-Colored Art Glass? Colored Fuse Glass? Recycled Glass?
How you want the edges finished? How much overhang you going to need to seat around it?
Especially with LED arrays dropping in price and available in more versatile forms, underlighting glass is becoming a practical and popular option. Is this to be part of your design?
You will probably have to have the metal bases fabricated and plated, powder coated and painted? Or maybe you can buy some from someplace like Ikea and have them modified. Perhaps you can harvest some from a cheap thrift store piece? Obviously you will need enough support for both the length and width of the glass but with structural members as small as possbile?
You will no doubt have to do some work to get a level surface to start. Something like Abatron's epoxy would work well. You can color it and tool it with standard tools. You will be able to drill through it and the stone to anchor your supports.
Too hard to estimate cost until you fill in some blanks.
Meanwhile, take a look online for solid glass countertops and tabletops for some great looking ideas.