Soloed my CW to 60 with a man-at-arms companion too. Lots of potion chugging involved, but it's completely covered (and more) by the income you make. As long as you make the right decisions with your CCing it should be fine. I had some trouble in the last section (around 55-60) but most people in that zone are looking for…
I've been really curious about this as well! I ended up assuming the big chest was a reward for doing the dungeon during one of those timed bonus events where you get an extra reward for doing a dungeon at a certain time...