I'm going to inspect a 1988 Volvo 740 GL tomorrow,
the ad says he's the second owner, has a full log book and
the car has been serviced regularly every 6 months.

The car exterior and interior looks good from the pics,
and the listed price is A$3000 (=~US$2700) so I'm pretty keen.
But the thing that I'm worried is the maintenance cost, I know
that maintaining an European car would be more expensive than
a Japanese car, so my question is how much in dollar terms
have you spent on your Volvo yearly? Spare parts and labor costs

And bear in mind that although I do want to, I don't have resources
(tools and time) to fix my own car as I am very busy in my job now.

 

I don't find the prices for parts to be too bad. Especially if you order them from rockauto.com. The old Volvos are pretty reliable and don't break down as much as many other cars either. They are also easy to work on and plenty of room under the hood so that would help keep labor time down.

 

What I mean here is if something breaks down then I will
hand it to a mechanic to fix it, not by myself

 

I said they are easy to work on and plenty of room in engine bay to keep labor time down which will cost less at the mechanic shop and parts aren't too high. However if you take it to a shop I would take the parts with it so you won't pay extra for the shop's markup which can be quite extensive.