Actually Jon is right, the true cost of a combustion analyzer is in the replacement of cartridges or service. If you buy a $300-$900 combustion analyzer expect to pay a few thousand over years of ownership. Not to mention anything less than $900 (new prices) will generally not display both Co and O2 (usually one or the other), and is junk. All combustion analyzers have to figure O2 and Co2 in order to know the other (as well as draft pressure and some other numbers). Make sure when you are reading the specs it does both calulations and display. Often times customers get confused that if a combustion analyzer figures something then it should display it, and that is not true. There are other features out there that are important as well, if you need them. Draft pressure, stack temperature, O2, Co2, Co, NoX, data logging, IR printer, and the list goes on. Out of all of those NoX is very important. If the gas hits the Co sensor and the NoX is not filtered out, the Co figure can be off by as much as 25%.
As my previous post stated I highly recommend the C157 UEI, because of the NoX filter, if you are looking to save money up front go with the Testo 327-3. The 327 also has a built in NoX filter, and right now the 327-3 is cheaper than the 327-2, they are $250 off through the end of the year. The -3 comes with an IR printer, the -2 does not. The 327-1 does not come with the printer or a case, the -4 adds on a smoke spot tester to the -3 kit. For the C157 you will be in the $1500 ballpark for the kit. In the $1350 range, or so, for just the meter, probe and case. The 327-3 is in the $1k ballpark, and so is the 327-2. The 327-1 is right around $900. The 327 has cartridges to replace and a 3 year warranty, the UEI c157 does not use cartridges and has a 5 year warranty on the unit and the Co sensor, and a 10 year warranty on the O2 sensor.
Just remember test & measurement equipment are not like power tools, if you buy cheap that is exactly what you get, or your cost of ownership over time is much higher. With power tools, providing you stay with the larger name brands, it really is the Chevy vs Ford arguement in many cases.
JJ