For low Gauss measurements, yeah the $3 Hall sensors available are great, but if you want a serious DIY meter that can measure up to 20,000 Gauss, you are out of luck I'm afraid. They are out there, but you will pay about $600 just for the Hall sensor.
I searched extensively a few years ago. The only one I found that didn't cost an arm and a leg, and was able to come close to being "useful", was a part by Analog Devices: AD22151 @~ $4.00
The unit DS bought is a good unit, and I've asked the designer a few years ago what Hall sensor he used. The are not available any more, so he must have bought a few thousand from China when he had the chance. His unit sells for $300 I believe.
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