I just played around on a couple of rent a car sites and none would allow you to drop off in Alaska.
I frequently rent cars in the lower 48 on business. And drop them off in other locations, for example rent in LA drop off in Pheonix. They chage me about a $1/mile dropoff fee. On that and with the info from other posts, I guess you should expect a $3000 dropoff fee. Pretty pricey!!