Is it expensive to fly to Austria?

It depends on where you're flying from.

Go to Google Flights and take a look. Or Skyscanner.

I could fly to Salzburg for £39 - often less. I don't want to but I could.

This is the best search engine for flights:
https://www.kayak.com/flights

That’s easy to find out on your own. I use Travelocity to check flights and fares. Depending on where in Austria you want to go, For us, Munich makes a good airport for trips to Austria.