There's a cost comparison sheet here: http://www.eia.doe.gov/neic/experts/heatcalc.xls
If you don't have Excel download and install Open Office to view it.
First of all I would suggest NOT purchasing bagged coal for such an operation, presumably you are going to go through about 15 ton a year for the greenhouse. I had a customer with similar sized greenhouse and that was about what he was burning. I'm guessing at the dimensions for the greenhouse. If you know the figures on the amount of oil or gas it would require to heat such a building you reverse calcualate for coal. You can purchase here locally for about $110 - $130 ton in bulk and then have it shipped, I'd suggest simply buying it by the tri-axle load. Or if you have a large enough truck get it yourself, be well worth the trip if you have a large dump.
In his case he had a crushed rock floor and a hot water boiler. He ran pex tubing through the floor.
He told me he couldn't possibly afford it without the coal, you of course are going to be paying more than him because of shipping charges.