Up to 2000 poly's for
AI aircraft ::)
Nice
exterior models can easily run 10-20,000 on multi-engine and/or complex gear aircraft, up to 40,000 depending on desired details which include cockpit and cabin features.
A good, detailed
VC which includes cabin can be 10-20,000 if it includes wings, engines, and gear views (like on high wing aircraft). :o
Check the xxx.mdl files of default aircraft and compare them to your resulting xxx.mdl sizes. My aircraft .mdl's run about the same as the larger default aircraft, yet I usually have 68-78,000 poly's total interior and exterior.
It really depends on the kind of aircraft you want to do, and its intended use. High poly counts with optimization, frugal yet practical use of polys, and a balance of optimized textures can still be frame rate friendly, but may hurt you when you pile on AI and major airports, and in multiplayer sessions.
Regardless of size, optimizing your model and textures should always be part of the design process.

To establish meaningful averages, categories of aircraft, intended use, engine classes, gear complexities, and VC/cabin content must be considered. Part averages would probably be more meaningful for new designers to use as guidelines.
With that said, it's all eye candy, and what most simmers want is a great panel and great flight dynamics. These two things require almost zero polys. ;)