Specific Bonus %'s on all items
Only decorations have a multiplier. Buildings just earn you cash and if they come with characters, premium tasks.
Forum Discussion
OP, if you don't want to risk accidentally selling your premium items as you store them, perhaps this wiki might be of help?
http://simpsonstappedout.wikia.com/wiki/The_Mayflower
Each premium item page should have info on the payout increase
http://simpsonstappedout.wikia.com/wiki/The_Mayflower
Each premium item page should have info on the payout increase
