you should be able to get the 20lb bottle and regulator for about $300 total, use the web and do a search...tons of places. it will cost you about $30 every two weeks for the C02 based on useage, simple tables will tell formulas to use.

My grow room is 19 x 15 and I have intake air and a carbon filter for exhaust air + C02.....exhause every 15 minutes, for 10 minutes, then add C02 for 15 minutes in between on a timer.

Should work excellent.

Check out my grow log....it has a list of items I am using.