Here is another link to a relatively recent press clippping in...

Here is another link to a relatively recent press clippping in 2016 that is interesting from the perspective that it confirms that Southwest Airlines is going to retrofit all 583 of their Noeing 737NGs with sensors to monitor their engines bleed systems to supplement their health monitoring systems. In this article they also refer to costs for different types of sensors. The articles quotes Ambraer as saying that the comparative vacuum sensors cost a few hundred dollars per monitored area. What is not clear is what they mean by monitored area, is that a particular weld or a wider area? Suffice to say that from this you can run some numbers. For example, it takes around 70 of SMN's sensors to monitor cracking in one wing, so it is 140 sensors for both wings. If they got $300 for each sensor, that would equate to $42,000 per plane. With 583 of the 737NGs in their fleet, that equates to $25M. Whilst this assumes they get $300 for each sensor, it maybe less if it is several sensors for each monitored area. These numbers are merely for one model aircraft from one carrier. When you multiply it out for the whole market you get to sme very large numbers. This is the link to that article -> http://www.mro-network.com/maintena...g-advanced-airplane-health-monitoring-sensors