I have used tracers, up to $800 in cost, with mixed results. Maybe it's our local soil, maybe it's my technique (or lack of it), but all of them seem to require a lot more 'art' than 'science.'
That said, they are a valuable tool, a real help, in figuring out what is going on.
I've also found it a help to use marking paint to indicate solid 'hits' .... sometimes, the pattern of dots will help you to see a pattern, and find your problem. I found a paved-over, faulted junction box this way. (Also - in true "Murphy's Law" fashion ... I was parked right over the box!)
Still, the best help is 'additional information.' That is, first-hand knowledge of the installation, or photos of things during construction.
For example, I once was chipping the concrete around a pool in order to bond to the rebar - and was not finding it! Watching a video the customer had made of the pour, I was able to see - you guessed it - that I was chipping right where the helper had stood - pushing the steel to one side. Murphy again!
Still, I am able to sort out 90% of my tracing issues using a $30 'circuit breaker finder' on hot circuits, and the $130 Progressive/Tempo unit on dead ones.
