Penobscot County is going back to the drawing board with its proposal for a new or expanded jail in Bangor, weeks after the county’s three commissioners backed a plan to build a new, 300-inmate jail to replace the current one at an estimated cost of $65 million. The commissioners Tuesday told a jail advisory committee to come up with a proposal for a new or modified jail that would cost between $20 million and $30 million. Chairman Peter Baldacci said that feedback from municipal leaders and the public caused commissioners to conclude that taxpayers would not support such an expensive facility, which voters would have to approve. “I think there was a consensus that [the $65 million facility] was the best plan if money was not a factor,” he said.