Oxycheq has a model called El Cheapo for +/- $100.00 if you have the technical skills to assemble the unit (soldering iron, etc.)
I am a big fan of this particular model.
I added the expedition sensor cap to mine along with a flow restrictor and QD fitting and in that configuration it will give very accurate and reliable readings with a minimum of fuss. It uses a commonly available external sensor, so changing the sensor is easy. I also use the sensor saver cap on mine and I'm getting 3-4 years out of the sensors, and I put mine in an Underwater Kinetics 309 dry box - it's the perfect size to hold the case, sensor/chamber and tubing. The analyzer also uses a 9V battery and I've replaced mine once in 8 years of use.
The soldering is pretty simple stuff and the hardest part of the whole project is cutting the hole in the case for the LCD display - but if you have access to a dremel tool or a drill (for the corners) and a coping saw (to cut between the corners) it's pretty easy. Plan on 2 hours if you've done one before or have help from someone who's done one to maybe 3-4 hours if it's your first.
Current price for the kit is $110:
EL Cheapo II Analyzer Kit : OxyCheq - DiveOps, Analyzers and Scuba Diving Equipment
The Expedition sensor cap is another $20, $40 with the flow restrictor and QD fitting - not a must have but nice - and the sensor saver is $4.50. The cost of the dry box is $25-$30. So for $150 you get a very functional and easy to use analyzer and for another $30-$35 you get all the perks. At $185 it will still be a lot less than an comparable analyzer costing $300 or more.
A new R17D sensor will cost you $60 to $80 as as noted above with the $4.50 sensor saver I get 3-4 years out of them before they start failing to calibrate or read properly with 100% O2.
The qualifier here is that once the sensor starts to get old, I'd advise calibrating periodically with O2 to ensure they are not voltage limited as if that happens, they can appear to calibrate correctly on air, but won't read properly with 32% etc. The other indicator here is that if the mixtures start to be consistently off, it's probably the sensor not the mixes themselves. It's pretty obvious once you know what you're seeing and understand what's happening.
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Personally, I am not a big fan of the stick type analyzer as the pressure at the sensor effects the reaction and thus the reading varies with how much the valve is open, how tight the seal is against the valve, etc. I think a Tee held to the valve (as in the basic El Cheapo II model) is better than the stick approach, and a sensor chamber with a flow restrictor to deliver constant flow from a QD fitting is even better.
Temperature and humidity also matter, but there are two simple ways to calibrate:
1) a simple chart showing temp and humidity with the O2 percentage to use (for example it might call for 20.6 rather than 20.9) and calibrating with the sensor open to air works fine, and
2) calibrating with the sensor chamber attached using a known source of 21% and setting it to 20.9
Some of the more expensive units sense temp and humidity and automatically calibrate, and while that's nice, it really isn't much of a time saver. It only takes a few seconds to wave the sensor around in open air and set the percentage based on a chart stored in your dry box.
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If you are going to spend $350 + on an O2 analyzer, then my opinion is to just go all the way and spend $600-$700 on an O2 and helium analyzer.
