I believe a single ticket now costs 124kr so 248kr return. For 291kr you can get a ticket that is valid on all public transport in Oslo for 24 hours, including unlimited uses of the Vy train to and from the airport.

You should be able to buy this on an app. Supposedly a convenience store sells it as a physical single-use ticket, but I was unable to obtain this, so the staff at the airport station gave me a smartcard for free (normally chargeable) which I used to buy the ticket at a machine.

Note that it's not VY (vee why) but Vy (pronounced something like French "vue") which is a Norwegian word meaning vision.