My immediate thought on this is that it's going to be extra-challenging for a few reasons:
1. The ID is normally on the tail (er, vertical stabilizer), which will necessarily be moving perpendicular across the field of the camera, rather than more-or-less directly toward or away from it.
2. Coupled with the higher speed you're likely to see, there's a far greater chance of motion blur obscuring the ID.
3. Add to this, LPR on cars can normally be focused on a pretty small area as cars go through a "choke point"; with planes, there's a WIDE range of heights the ID may live at, meaning a wider camera view, meaning less detail.
4. LPR is generally aided by the fact license plates are made to be highly reflective, with high-contrast letters, which allows the function to be aided by relatively low-power illumination/flash, ie. it takes very little light for the plate itself to be brightly lit, while the letters aren't. Airplane IDs are also high-contrast, but the surfaces aren't generally that reflective, requiring more light (brighter flash), which in itself would be hazardous to other traffic.
5. I would expect range from camera to subject would be much greater than typical with LPR, meaning weather (fog, rain, snow) is going to be a greater impediment than with LPR.
Given all this, I'm not surprised that a *workable* solution would be quite expensive. Keep in mind that true LPR was traditionally quite expensive until just recently as well... takes a while for the economy of scale to kick in.