Yes 18000 poly's is high for a player model in a game....
The maximum amount of poly's that one mesh can contain in GDK is 20,000. You can have more than that in an object though, by using limbs, but per mesh, its 20k..
The average for most games is anywhere from 500 to a few thousand on average. Some newer AAA games have higher poly models, but they also have much more technologically advanced engines behind them capable of rendering that many poly's efficiently... GDK isnt without alot of work setting up LOD and frustrum culling.
You should try to make you models use as few poly's as possible while maintaining the detail you need. Its important to use as few as possible because you may have many different objects onscreen at one time, and if they are all in the 10's of thousands of poly's, your program will slow to a crawl.
If it ain't broke.... DONT FIX IT !!!