How much would it be to get a valve cover gasket repaired?

should not be more than an hours labour (usually around £30 mark) you could do it yourself if you got a set of spanners/sockets as rocker gaskets are an easy diy job, undo the retaining bolts ontop of the rockercover, remove any pipes attached (remember where they go if more than 1, could use tipex to mark) when there is nothing obscuring it from being removed give it a tap with a hammer and replace the gasket, do not over tighten the gasket bolts, check tightness before removing to give you an idea of torque