the gasket around the moonroof tore. It is ok, so long as I do not open the moonroof. What would it cost to replace that part?

Can't say how much the repair will be or IF it will leak as it is .. However if you don't use the "moonroof " often and 'leaking' gets to be a problem, you may consider sealing it up and disabling the power circuit......... Just an option.