If its the Antenna that needs replacing it is around £25-£30. But sometimes this can be a board fault, which unfortunately wouldn’t be repairable.
What i would suggest is to book it in for a diagnostic, by following the link below, then we can have a look, and check the fault. If it then needs a new Antenna the diagnostic fee would be included, so you wouldn’t have to pay it as an extra!
Samsung Galaxy Tab 3 Repairs