The cheapest is the iTP EOS A1, you can probably get it for like $21 and it does 1.8 lm so that's pretty low, but uses RCR123/16340. Issue is it was discontinued like 1 month ago.
Email Stanley at HKE, shipping is included and he can do a few dollars less if you mention you are a BLFer.
http://www.hkequipment.net/?sp=&p=6&cat2=189&cat1=36&cat0=1&id=1028&cat1=36&cat0=1&new=&more=&s=12130b242328c0a7f3d3627886f2ff43&lang=en
Check out the Ultrafire UF-H3, the low should be very low, the thing is that it is a floodlight so at the lowest setting it definitely would not harm your dark adapted vision. Issue is it has somewhat bad 110Hz PWM (flickering like your old CRT). But it is an immensely useful light esp for your use. Dinodirect is having a 3-day offer from $32 discounted to $25, shipping incld. Search for the thread.
You can get the Zebralight SC600, pretty nice branded light with very high quality machining. 2 different users told me the machning is even better than Surefire. The special feature of this is, this is the smallest XM-L + 18650 light ever available, very good quality and natural type III (hard anodized) anodisation, still below $100, perfect beam, and probably the brightest EDC you can ever buy. I have measured the budgetlight "equivalent" ie the Uniquefire UF-2100 (but that's 20mm longer), somehow it is not getting the OTF lumens out and mine has a slight flickering problem already after 1 month of use but it still works. People will complain that it's expensive, but my opinion is just splurge once for a good EDC light + night light that you will use almost daily for the next couple of years, coz that is the single most used light and it is worth it.
High: H1 500Lm with 750Lm turbo in the first 5 min (2hrs) or H2 200Lm (5.9hrs) / 330Lm (3hrs) / 500Lm (2.1hrs) / 4Hz Strobe
Medium: M1 65Lm (18hrs) or M2 21Lm (50hrs)
Low: L1 2.8Lm (280hrs) or L2 0.1Lm (80days)
^^^^^^^^^ Note, you can have a choice of 65lm which is often the lowest mode of a normal budget light, and then you also have a choice of 21lm which a small number of budget light can reach, 2.8lm which i think no other XM-L budget light can reach, and then a 0.1lm mode of 80 days.
And doing all these low modes with absolutely no visible PWM and a side clicky.
http://www.hkequipment.net/index.php?sp=&p=6&cat2=105&cat1=48&cat0=1&id=911&cat1=48&cat0=1&new=&more=&s=56330e21b8effb47a239580358f732b5&lang=en
If you do not need the power, get the Balder SE-1. 3 lumens for 50hrs with a 14500. http://www.cnqualitygoods.com/goods.php?id=1297
14500 - Trustfire Flames (protected) tested with a capacity of about 750mAh on the hobby charger. A pair is less than $6 shipped.