It's £28.50 for a return for the car on the ferry from Oban to Mull which is not bad + £7.70 for each passenger. It used to be close to £90 for a car and 2 passengers.
I've taken the bike from Oban a couple of times. Give yourself plenty time to park as it can be a nightmare and depend where you park could be around £10. A lot of places that used to be turn up and park are now charging.
Parking at Tescos used to be fine because it was never full but that is frowned upon now.
There is good parking at Mossfield shinty pitch which is a short bike ride, to the ferry and worth a look.
Shouldn't be any issues parking there.
All the best.