There's a saying that I can't quite remember, so I'll just say it in my own words: reward people based on a metric, and that metric becomes their main concern. Worse still, they will learn to game the metric.

Building a new system that doesn't have a baseline cost? If the cost saving bonus is based on a reduction in existing cost then you don't get rewarded for a new implementation. The business don't have a baseline cost yet. So what do you do? Build it to run at a higher price, run it for a few months and then cut costs.

Also, is the reverse true? Deny bonus if costs rise. Because if it is, then the engineer has to prove that the increases are directly related to increased customer activity. But what if increases aren't relative to customer spend? Etc