Okay, I didn't get a calibration guarantee with the Weber, I can't replace the batteries on my Weber and I got my first Thermopen from ebay at a current cost of £28.00
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http://www.ebay.co.uk/itm/Splash-Proof- ... 7171920%26)
They (ETI the makers) say that they are refurbished models, I have yet to find a difference between the ebay version or the other 2 I have one a gift the other from the Harry Soo course.
Look at it in a purely practical way. I would not buy a BBQ from a company which has built its international reputation on building the best temperature registration device available to those on a restricted budget (you can easily spend hundreds of £/$ on temperature recording devices as you can cars (all get you from A-B in different levels of comfort

)
Working on that premise why would I buy thermometer from the manufacturer of the best BBQs. The expression Jack of all trades master of none is an obvious comment.
Okay I accept that maybe a couple of degrees out one way or another may not affect your cook, but you are buying a thermometer from either;
A) a company which made its name from building the best bbqs around, then branched out into everything from oven gloves to thermometers, including pepper mills & kebab racks OR
B) A company that sells Guaranteed Certified thermometers. A company that guarantees its product and does nothing but build award winning, world renowned, industry standard heat registration devices.
The concept of buying the best you can afford for a lifetime usage is (IMHO) obvious. But the choice is yours. Let us know what you bought and where you saw a Thermopen at £165 at least one of our members would be interested at that!!