I am having 8 recessed cans installed in my kitchen. What is a fair labor charge for this? $50 a can? $100 a can?

It depends on if new construction, or remodel.
Is wiring already from switch to ceiling area?
Cans can be from $6 to about $15/more or less, not counting trim, trim can be VERY expensive.
$80 an Hour is not a bad price per hour, depending where you live.
2-4 hours could be expected, if exposed.
If work has to be done through attic or difficult areas, add more money.

$120-$150 per can (complete installed) is a range for old work.