Grab would be a nice solution bcoz you don't have to worry about parking. All delicious local food in Penang is situated in a busy street in the city and it is limited parking space available.

I think it is consider cheap already. You can try Dacsee and MULA to compare the price. Airport was located at South East of Penang Island while Double Tree was located at North West of Penang Island. Different time the price different also coz it calculate based on traffic too.