I must admit I am not sure about costs, but some operators do offer APS upgrade courses.
I have been involved with teaching the APS course for the last year, and one thing I would say, is the content of the course is a lot more relevant to the modern airline environment than the previous course, and of course it is regulated by the CAA.
Another advantage, is the course is designed and run in the same manner on how the airlines train, i.e. competency based training and its purpose is to develop skills in all the competencies to a competent standard. When cadets come through the process, if they dont achieve the required standard they won't be awarded the APS certificate. Therefore, for comments to be made about the airlines dont 'give a toss' on what you have, I would have to disagree, as having this qualification says something the standard you have achieved. As for airlines that have never heard of it, well any competent airline who is recruiting and familiar with Part FCL and licensing will be familiar with FCL.735.A which is where the APS course details can be found.
Regarding assessments with the airlines, as above, because the student pilot has been subjected to the competency based training framework, I believe the skill that would be been developed would be an advantage when being assessed in an airline.
Hope that helps.
All the best with what you decide.