The overdrive on the MGB is activated electrically but what it is hydraulic unit. It uses the same oil as the gearbox. The unit has it’s own oil pump to make it work. It is a simple system but one must really know what he is doing in order to fix it. Usually all parts in have to be replaced including the pump nad check valve onece it fails. Actually that is the best way to go with it rather than having to get in there again.
I find that $3000 is a bit high for this. Try to see if you can find a complete unit someplace and just swap it out. It is not that difficult seeing that it is bolted to the rear of the gearbox and the latter does not have to be dissasembled in order to separate them.(I hope I am making sense, I think a lot faster than I write)