400W is a sensible starting level for a 6x8, for overwintering and for plants that aren't hankering for direct sun. You could comfortably use twice as much if you are looking to get some growth and flowers from your plants. Depending on your need for temperatures, you might be better to line the whole greenhouse with insulating sheet. Something like white polystyrene sheeting would be good, it needs to be white or reflective (or possibly transparent). 400W isn't enough to maintain the sort of temperatures I think you will want for those plants even running 24/7, but with decent insulation, you might do well with extra heat. Possibly the white walls will reflect back more light from the lamp than they block from outside. Not sure on that though, it will depend how much direct sun you block.

Multiply the power in watts by the number of hours a day you run the lights, then multiply by your unit electricity cost in cents, divide by 3,000 and you have your approximate monthly cost in dollars.