First of all: I assume this thread should be in the proper geographic subforum? It's not a BA or BAEC specific topic.
Sweden is a very expensive country.
Compared to 3rd world countries - perhaps. However Stockholm isn't that expensive in global comparison for a visitor. Honestly speaking something like London is more expensive to get around in etc.
The Arlanda Express from the airport to the city can be around £60 return (cheaper at weekends and for couples). Need to consider these costs too as they build up quickly.
I advice against going with Arlanda Express. No need to waste money on it. It's the same thing as with HEX - only use it as a last resort. Either take a direct airport express coach (Flygbussarna) or use regular public transportation, which includes the option of going by rail or rail + bus.