In 2014, it was a "$450" upgrade/option if you checked the box. Checking the box meant that you got the benefit of the value of the OE stock system which would have been applied against some cost differential for the value of the 'sport' upgrade. I get that you didn't want all of the stuff that was included in the $5K package but the same would have been true for 2015. Essentially, they were bundling a $500 exhaust upgrade with $4,5k in other upgrades. You had to know Mercedes wasn't really going to sell anyone a full exhaust system for $450, right? 2014 owners who failed to check the box would similarly be hit with the same $3K price for the exhaust if installed after the fact.
The fact that you didn't want all of the extras doesn't mean the actual costs or logic changed. I fully understand why you opted to skip the upgrade -- but it sounds like you may not have thought the next part out fully. Even an Armytrix exhaust system for the 45 runs like $2,600 (pre-tax). As we've all seen, AMG doesn't mean cheap...quite the opposite.
-Eric