I bought a Sunding 548 4 years ago for $5. It kept distance correctly once I adjusted the multiplier: the 'manual' recommended a setting that turned out to be about 2% high compared to the markings on city bike paths, possibly because the manual may not have accounted for slippage. The clock gained about a minute every week - 1 part in 10,000. So I was happy with it. I broke it through careless handling then bought the current version for $3 on eBay in January. It seems to keep time nearly perfectly and the 'manual' setting for the multiplier was closer. The vendor didn't ship a manual; I found a PDF on Sunding's website.

Someone offers it for $108 on eBay: I wonder what's up with that.

I didn't want to worry about batteries in the sensor and haven't minded the wires so I use the wired version.