It's 5 diamonds per single goods last time I checked, so any additional diamonds in that price are likely due to other expenses that the payment would cover.
As far as motivating all buildings goes, the option does quite a lot for that cost. "It instantly boosts your buildings" is the summed up plain idea, yes. But it basically gives you an unlimited number of "help by other players" and in the way you want. Instead of dropping messages to your guild asking for a motivation or a polish or something, you can do all of this yourself, and still have the availability for your guild/friends/neighbors to do some action in your city at a latter time.