Well, I suppose it could be. It all depends what is happening.

If your mechanic is over tightening the manifold nuts, then this could
cause the gasket to distort and fail. Also, if he is not cleaning the
old gasket off the mating surfaces thoroughly, then this can be a
problem.

If the manifold is warped, then you can probably have the face
re-surfaced - even lap it on a surface plate with emery cloth, for a
fraction of the cost of a new one.

Hard to tell with Volvo. Phone your nearest dealer, ask for 'Parts'
and ask them. Make sure you are sitting down at the time.

But a secondhand one from a breaker's yard would be a realistic
option. Prolly about 30 quid.

How certain is it that the gasket has failed? Presumably you are
suffering the symptoms of an intake leak - typically uneven idle,
maybe a bit of hesitation, perhaps a whistle that is often higher and
louder at tickover. A common problem with these cars at this age is
leaking injector seals (if you are injected), which will cause similar
symptoms. There are other causes of an air leak, too.

Nothing I can think of so long as the bolts are properly tightened. I've
never had an inlet manifold gasket fail though so it'd have to be very
warped, they should be able to measure this easily.

First thing to do is stop taking your '89 to the dealer, that's ridiculous,
you can get much better service and prices at an independant shop. I can get
manifolds at any of the local U-pull yards for about 20 bucks, you should be
able to find something near you, if you have to pay more than $50 just let
me know and I'll go pull one for you.