I just got a $30 Harbor Freight (item 47257) six inch digital caliper, from the noodle factory, partly as an experiment, and ultimately to give to my 9 year old nephew.
It is functional, albeit more crudely made and less accurate than a Mitutoyo (which costs four times as much). The random error of the 47257 seems to be about 0.002", largely due to head looseness. I tightened the head gib, and the error dropped to about 0.001", which is what HF claims. I don't know how well the gib adjustment will hold - the two adjustment screws seemed a bit loose.
The clamp thumbscrew (that locks the head to the bar) was bent, so I straightened it with an adjustable wrench. No idea how it was bent, as this takes some force. The caliper came in a plastic case, and shipping damage was not apparent.
The head slid reluctantly on the bar. This appeared to be due to a coating of blackish grease, which easily wiped off. But still there was too much drag and slip-stick. It turns out that the bar was precision ground but not polished, leaving grind marks perpendicular to the bar, so the grind marks were slowly filing away at the gib, which is brass. Not good, so I polished the grind marks by hand with crocus cloth. This helped immensely. I suppose that crocus cloth wrapped around a metal sanding block would do an even better flat polishing job.
After cleaning all the nasty grease away, I lubricated the caliper with pure lanolin (used for instance to lubricate brass musical instruments). Pretty smooth now.
The manual says that the caliper requires a SR44 (silver oxide) battery, but it came with two alkaline cells. Another corner cut.
I will use this caliper for a while in the shop and see how it goes.