I don't remember the figures but I can offer some insight on this.
Having worked for a company that flies NG's and studied the 170 as a possible addition it broke down to this.
Per hour of flying the 170 was cheaper but when you add in the cost of a new type,training,parts, learning curve ect.. it was cheaper to stay with a smaller version of an NG (-600) than go with a new type of smaller a/c. Just the initial cost of that new type was an up front 54 million, that's before you see an a/c. Not the answer exactly but I thought it might be interesting.