Governor Tom Wolf's latest state budget proposal calls for a severance tax to be paid by companies drilling for natural gas in Pennsylvania's Marcellus shale formation.  David Spigelmyer, President of the Marcellus Shale Coalition, says the governor should realize that such a tax, coupled with burdensome regulations, raises his members' cost of doing business.   But some supporters of the severance tax don't expect drillers to abandon their  investments in Pennsylvania if it's enacted.