Are the guides a must have? That's what drives the cost up. I love this company :
http://www.sherpaexpeditions.com
But their self guided trips are much cheaper than guided. Even the guided, though are cheaper than back roads.
With the self guided you still get accommodation, restaurant recommendations, activities, luggage transfer, and emergency support. Honestly, we prefer it due to the flexibility
I've also heard good things about this company
http://www.atg-oxford.co.uk
But we are doing our first trip with them this summer, so no direct experience. Same deal - guided trips cheaper than back roads, self guided much cheaper still.