Are you asking how much would it cost for you to do the work yourself? Or are you contemplating hiring it out? If it is a DIY, then it will be the cost of two sheets of drywall of the appropriate thickness, a roll of drywall paper, a handfull of drywall screws and a small pail of mud. You can price these yourself at your local home center. Now if you are asking for an estimate of hired work, this is something which cannot reasonably be determined via the internet. There is a vast difference in labor costs from locale to locale. Additionally, this small a project might be difficult to interest a professional. You probably should check with two or three 'handyman/home repair' type of individuals. (Get references first.)