I spotted this list http://www.foodnews.org/methodology.php
a year or two ago and try to mostly buy organics from the "most contaminated" and traditional/cheap from the "least contaminated." Most Contaminated
: peaches, apples, nectarines, strawberries, pears, imported grapes, Sweet bell peppers, celery, spinach, lettuce, and potatoes.Lease Contaminated
: onions, sweet corn, asparagus, sweet peas, cabbage and broccoli, avocados, pineapples, mangoes, kiwi, bananas, and papaya.
In my world, that means apples and peaches are ALWAYS organic, but I don't worry much about mangos or avocado. And in our local "farmer's market" (think warehouse supermarket of awesome food) they only sell organic bananas, for about $0.05 more per pound than the regular grocery store sells conventional bananas, so I buy those organic even though they're low-risk.
For items on neither list, it depends on the price & availability of each, and on whether I think my toddler will be eating a lot or only a little bit of the food in question.