Tim, what were the rental costs on the 100-400 II? Locally they are $50/day, $37 for an additional day, $50 for a weekend, and $150/week. All in Canadian pesos of course.
I rented the 100-400 II over the holidays and they had a discount for that period. It was $141 for two weeks, it would have been $22 less, but I included the all-risks insurance so that if it got stolen, or lost, or dropped, its 100% covered. We were on a trip during part of the rental period and I didn't want to have any worries about theft from a hotel room or the car. I used lensrentals.com - good people to deal with.