In fairness to all, we don't discuss pricing on the Wall. Thanks for your cooperation.
I noticed you said "another company replaced the furnace a couple years ago". Did this replacement include a new roof jack? If not, your customer may have a venting problem. Coleman had a lot of "issues" with the inner pipe becoming perforated and allowing flue gases to be recirculated back into the combustion chamber. We run into a couple of these every year the way it seems. Pilots that won't stay lit, smells, poor looking flame, CO are all symptoms that occur when you have one of these go bad.
Get up an the roof and check it out with a flashlight. Another thing we have found on these POS furnaces is that debris and rust from the flue will fall down and acculmulate on top of the flue baffle, which is right at the top of the HX. This causes a restriction in the flue and can lead to some really wierd things going on with the furnace. Check that out too. The fix for this is to drop a shop vac hose down the flue and try to suck it out. If that won't work due to the debris being a solid pile of rust, the only thing you can do is get a long piece of pipe or a broom stick and push the baffle down to allow for sufficient exhaust area. This obviously reduces the efficiency of the furnace but it's about all you can do short of pulling the whole thing out and cleaning it.
Probably the best thing you could do for your customer would be to sell them a new ThermoPride mobile home furnace. These are without a doubt the best made furnaces on the market for mobile and modular homes. The Coleman is an accident waiting to happen.