Bargain vs moderate

Hello. In the following multiple-choice exercise:
“Here is a wonderful opportunity at a … cost to visit the truly remarkable island of Cuba.”
A cheap B moderate C bargain D small
The answer-key gives B moderate as the only possible answer. However, I had done a search in google and found quite a few instances of “bargain cost”, so why is this incorrect? As usual, thank you for your help.

Monica, notice that in the Google results, the words “bargain” and “cost” are usually not part of the same collocation. You find things like:

Will [size=75]Noun Phrase[/size][that bargain] [size=75]Verb Phrase[/size][cost you more than expected?]

It’s common to say “bargain price”, but it’s unusual to say “bargain cost”.

Where you happy with the answer you got here?

forum.wordreference.com/showthread.php?t=984664

Yeah, I had the feeling it was only a matter of collocations, I just wanted to make sure. Thank you.