I am a little dismayed at the false impression given about either the Alaska Highway or the Marine Highway wether "tongue in cheek" or not. I have traveled both and never needed tools or a spare on the Alaska Highway and traveled on 4 different legs on the Marine Highway and found the accommodations more than adequate including a berth for one overnight trip. The ferry is comfortable and has great cafeteria style dining.
On the Alaska Highway we never had a close call with gas, just ride on the top half of your tank, never had a flat tire, and the only damage in two trip on the Alaska Highway was a stone chip in our TV.
We are planning a trip again in 2015 and will travel on both "highways".
To answer the OP question on our trip from Maryland in 2008 we traveled about 14,000 miles and spent about $12,000 dollars which included food. We did do a number of tours, but for many used the 2 for 1 Tour Saver book. Our gas mileage was about 16-17 mpg.