Mahle may have discontinued 6.9 pistons, but you should be able to still get them from Speed-Pro or Silv-O-Lite.
What was the content level on the reman 7.3 IDI? If it includes fuel, $10K is a pretty good price. The remanufacturer I work for sells 6.7 Fords and DirtyMax long blocks (no fuel or turbo) for around $7K to $8K to wholesale distributors. How much they are marked up from that point for final sale to the end user, I can not say. However, you can rest assured that a large percentage of the cost is going to distributors, and not the remanufacturer. Remanufacturing a 40 year old platform such as the IDI is expensive and difficult. Core is getting rarer and in worse condition, often it will take 3 or 4 core engines to yield enough material for one good engine, and new replacement parts are getting harder and harder to find. That is why we won't touch the older IDI stuff. The oldest engines we reman today are 1st Gen HEUI DT466's, which came out in '94 and even those are falling off quickly. We had to reman a mechanical DT466 last year for a warranty claim. It took 4 core engines to yield enough parts to fully rebuild the damaged warranty unit. Needless to say, we lost our ass on that one.