Is $10,000 to replace the furnace a fair price?

I live in New England and recently had my furnace cleaned. My furnace runs on oil and is from 2001. It seems to be heating fine, but during the cleaning, the company told me that the heat exchanger was cracked and either needed to be replaced or the entire furnace needed to be replaced. They said that the screws in the back had soot around them which indicated that it was cracked and likely leaking carbon monoxide. I said it was not blowing soot out the vents and that we do have a carbon monoxide monitor which wasn't registering anything. After they left, I placed the carbon monoxide detector in the basement, first floor, and second floor (next to the heat output) and it just registered at 0. The interaction made it seem like they might be just trying to scare me into buying a new furnace. I don't know what those screws looked like last year though. I am seeking advice on whether this seems like a legitimate assessment of a cracked heat exchanger, what to look for if I get a second opinion, and if it would be a dumb idea for me to unscrew the screw and see what comes out while the furnace is running. My house is 3,000 square feet with 2 levels plus an unfinished basement, so only heating 2 floors.