That's ridiculous, I had a 1100cc Honda sport-tourer while at Ramstein back in the late 90s and it wasn't anywhere near that much. I paid something like $75/mo for full coverage, and was surprised when it really didn't drop much when we moved to Texas. I know Harleys are popular in Europe, and I know firsthand how much of a blast it is to ride a bike there (tell him to check out the Pfalzwald southeast of Kaiserslautern for some great nearby riding); but there's no way I pay would that much to insure a motorcycle!
Just to note, when I was stationed in the UK in the early 80s I did buy British insurance for it. He might check the places just outside base to see if they can provide coverage at a more reasonable rate.
Technically, USAA does insure motorcycles. It is through Progressive, but it is through them and I have two on my policy. You pay for them when you pay for your other vehicles and what not, but it is through USAA...
Cheers! M2