7018 AC will burn fine on DC the pics posted of stick welding with the 200DX was with some old 7018 AC rod i had.

Bottle fill price does vary depending on where you exchange, If i exchange my bottle here at the local farm store it is approx $51.00 w/sales tax for a 122 cu ft argon. If i drive 38 miles to North Kansas City it is $28.80 w/tax