Hey Charlie,

Most commercial tours are regional, usually operate in one country, maybe two. There are several around S. America. Not sure you'll find a company to guide you from AK to Argie, but there are a few BIG outfits that combine several countries in one tour. As you found; very expensive. $$$$$

If I had to pick a tour leader, Jay would be the guy!
http://jamminglobal.com/peru

But you can certainly pick up smaller tours as you go, shorter and more reasonable in cost. Use your bike or theirs. Or join other travelers heading in same direction ... but they won't treat you as paying customers.

If you need help and want to save money, probably best to hire a single individual guide who would ride with you and handle details.

Would have to be a special person: fluent Spanish (Portuguese if including Brazil), good mechanic, know border issues, cover insurance, find camping and hotels and of course know all the spectacular areas to explore suited to your riding level.

Most tours maintain a FAST paced schedule, cover a lot of ground in short order. This is OK for some, not for all. Many horror stories about tours.

Also, in a tour group LOTS multiple screw ups are COMMON and can change the whole plot of your tour, NOT giving you what you've paid for.

With a ONE ON ONE guide it's all about YOU and what you want to do.
A big plus, IMHO.

But no one can ride the bike for you, nor make you well if you fall ill. So in the end, you still have to do a lot for yourself. Perhaps consider a smaller trip to start off, covering smaller area, shorter term?

I would break-up an Americas tour into 4 separate trips:
1st: AK and USA (allow 2 months, Summer)
2nd.: Mexico, Cent. America (2 months, Fall)
3rd: Colombia, Ecuador, Peru' Bolivia (2 months)
4th: Chile, Argentina, Brazil, Uruguay ect. ect. (2 months)

Good planning!