Residential tax bills in Prince William County would go up by an average of $110 if a budget plan laid out by the county executive is adopted by the Board of County Supervisors. County Executive Melissa S. Peacor’s proposed budget of $912.6 million for the next fiscal year calls for a tax rate of $1.215 per $100 of assessed value over this fiscal year’s rate of $1.204 per $100 of assessed value.