There are a few costs the OP did not factor into his 'fag packet' calculations, and which have to be loaded on to each job the builder undertakes viz;
1. the builders' costs in running his van/pickup (vehicles are not free)
2. insurance costs
3. advertising/admin costs;
4. accountant's costs for his tax returns;
5. the cost of the builder's time in pricing for the 9 out of 10 jobs he doesn't get; and so on.
aka O V E R H E A D S
We don't know how much the SE charged for the calculations, nor how much per hour the OP thinks the SE has charged, and whether or not he thinks THAT is reasonable.
We hear a lot about rogue traders - not so much about rogue
customers.
The builder has to make profit. At the end of the day, there is a going rate for this kind of job as not everyone can do the work.
There is also an element of risk involved. He could turn up and find that the structure is not suitably prepared etc. Someone really needs to come and take a look at the job.
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