So far as I know, the maximum power for the heater is around 600W. It depends on the room and how the heater is installed, but I think the average power is probably around 300W. If that's true and you heat the table 24 hours a day and the cost per kilowatt hour is $0.15, it is easy to calculate the cost per year:
0.3kW*24 hours/day * 365 days/year * $0.15/KWh = $400/year roughly, or $30/month. (Someone should check my calculation.)
If the table settles down to 100W after it heats up, then it's about $130/year or $20/month. If it ran at 100% power -- a very unlikely case -- those costs would double.
So.... to really know the cost, you have to plug in the average watts the table is using.
Keep the table covered when not in use, and make sure there is no air circulation under the table. Some heating systems are designed with a closed box under the slate.
Not included in this calculation is the fact that any heat generated goes into the room. In the winter this is good. If you have AC on in the summer, this is bad. If you are on electric heat anyway in the winter, there is no extra cost to heat the table.