Of the three options, I think that either your second or third option is probably best. To break it down, what if the sentence just ended with the etc.? Then you would need a period after it both to end the abbreviation and end the sentence. From Lynch:
What if one of those abbreviations with a period appears at the end of a sentence? — do you use another period to end the sentence, or is one enough?
This one is simple enough: never double up periods. If a statement ends with “etc.” the period in the abbreviation does double duty, serving as the full stop to end the sentence. If, however, you need another mark of punctuation after an abbreviation, you can put it after the period. So:
This was her first trip to the U.S. (The period does double-duty, ending both the abbreviation and the sentence.)
So your sentence would end, "...cost of these events, etc." Now, I'm not sure what style of references you're using (eg. MLA, APA, Chicago), because this would affect how you actually place the brackets. However, no matter what style you use, remember that etc. needs a period after it no matter where it is placed in a sentence. (This is why I say both your 2nd and 3rd options are ok--they have a period after "etc", and could serve as different reference styles).