What drives the price
Length of pipe, how far the scale has closed the bore, and access. A short kitchen branch and a full lateral out to the main are different jobs by an order of magnitude.
Why access matters so much
If there is a usable cleanout, the work starts immediately. If there is not, gaining access is part of the job, and that is the variable most likely to move a quote.
Descaling against replacement
Descaling is normally a fraction of the cost of replacing a line, which is why it is worth establishing how much sound metal is left before anyone talks about excavation.
What we will tell you first
Whether descaling is appropriate at all. On a pipe that has thinned badly, taking the scale off can reveal how little is holding it together, and that changes the recommendation.
Why quotes vary so widely
Two companies looking at the same line can quote very differently because they are proposing different amounts of work. One may be planning to descale the full lateral and one only the accessible section. Ask what length is included.
When it is not worth doing
Where the pipe wall has thinned to the point that the scale is holding it together, descaling is not the answer and doing it anyway would be irresponsible. That is a camera judgement, made before the work rather than during it.
What it saves you against
Replacing a lateral is one of the more expensive things that can happen to a house. Descaling a line that still has sound metal is a fraction of that, which is why establishing how much metal is left comes before any conversation about excavation.
How long it lasts
Years, though hard water is still hard water and scale will begin again. The difference is that it begins from a full bore rather than from a pipe that is most of the way closed, which buys a great deal of time.