The manifold-to-cat section of exhaust nut is about three odd quid.
Only if you're stupid enough to buy them Vauxhall.
When I did my head gaskets, I replaced all of the head to exhaust manifold studs&nuts, plus the manifold to downpipe studs&nuts for £6. They're standard fasteners(the studs are the same as a Pinto for example), as are the screws the OP bought.
Well, yes, you are right on that one sometimes, as I found out at the back end of last year.
My policy is to replace any parts I need with genuine Vx ones to maintain my cars value in my eyes. Always have done that and always will. But sometimes it bites you on the bum, like the complete, top quality, direct from Germany, from OPel/Vx stocks, wishbones at £254 +VAT each, even with TC!
Still, they will outlast the car no matter how long I keep it, which I aim to be many years yet!