You're spot on with it all.
I realized I made a mistake, one I avoided elsewhere. ABC adjusts up to 30ppm/week - but in air, not water - so this is ppmv. As 400ppmv in air is about 1.3ppmw in water, the weekly impact of ABC in measurement of water CO2 content is then 30/(400/1.3)=0.0975ppmw. That is, if I haven't made another mistake, hehe. MUCH more reasonable than the 30ppmw error I was originally worried about.
As for the pricing difference between 1% and 3%, I'm not ruling out the possibility it's an artificial increase rather than more costly precision hardware. It works on this principle:http://en.wikipedia.org/wiki/Nondispersive_infrared_sensor
Since the light absorption of CO2 is tiny, I'm thinking it should be easier to detect at higher ppm, not less. I'd be curious if there are any visible component differences on the circuit side between the 1% and 3% versions.
I also visited the manufacturer's website, rather than co2meters. Where I was able to download the software included in the dev kit for free:http://www.senseair.se/products/software/uip-5/
And although I don't have a sensor, I was able to get this screenshot:
So it appears the $10 extra for the "custom" 1% sensor, is just a charge for co2meters hooking up the sensor and setting this for you. With the purchase of a dev kit (or maybe just a cable), you could do the same, probably even on the 3% sensor. There are other options too, it's possible you might be able to extend the range of the 1% sensor somewhat.
I happened across some other NDIR sensors too:http://www.allelectronics.com/make-a-store/item/CDS-1/Ndir_Carbon_Dioxide_Sensor/1.html?gclid=CKDLn7G7xbQCFQq0nQod_DgAww
Only good up to 2,000ppmv (6.5ppmw?). But at only $10, it might be fun to take apart and see how it works. Or try to modify it for a higher range.http://www.co2meter.com/products/s8-miniature-co2-sensor?gclid=CKGFw7fCxbQCFRRbnAod1WkAsA
$65, good up to 2% (20,000ppmv). Or 3.2% via UART only, with reduced accuracy. The sensing part seems to be sealed, with a gas diffusion membrane built-in. Could possibly be a superior choice, though I haven't read the datasheet fully enough to know if there's any catch.