Why it matters
Every hard floor needs a flat, dry, solid base. Laminate and vinyl plank in particular are unforgiving: laid over a base that is out of level, they flex at the seams, the locking joints wear, the planks separate, and within a couple of years you get gaps and peaking. That is why so many DIY floors fail, and it is why a professional quote is higher than a friend's. The prep is real work: removing the old floor, screwing down loose sheathing, patching, and pouring self-levelling compound where needed. There is also an honesty point here, and it is the one worth carrying into a negotiation. A quote that does not mention subfloor prep is not a cheaper quote. It is an incomplete one.
How to spot it
Noticeably different floor heights between rooms, or a step where there should not be one; tall or stacked transition strips doing a job that levelling should have done; springy or bouncy sections; gaps opening at plank seams in a floor that is only a few years old; and a floor that visibly stops at a doorway because carrying it further meant dealing with what was under it.
What it costs
Honestly: we cannot price this from a photo, and we will not pretend to. There is no defensible published Canadian rate for subfloor levelling, because the work is entirely a function of what is found when the old floor comes off. What we can tell you is that it is real, that it is routinely omitted from first quotes, and that on an older home it is one of the most common reasons a flooring budget grows.
What to do
Monitor, and ask for it in writing. When you get flooring quotes, ask each contractor to price subfloor prep as a separate line, including what happens if they find rot or a soft deck. Then compare.
Education and triage, not a home inspection. Casaroo can see the tells of a floor laid over a bad base. It cannot see the base. Nobody can, until the floor comes up. We flag; we don't inspect.