You are right on the rental cost. $84 per 5 days X 5 = $420. However, if I was going to spend 20% of the purchase price, I would just buy the zoom and after using it for a while decide whether to sell any of the primes. I use the 400mm f/5.6 a lot, and I almost always wish I had more reach. The zoom would likely be racked out to 400mm all the time anyway. The modest weight and fast focus of the 400 are hard to beat.