In all fairness,
@Missie this pant pattern is two pieces with buttonholes for drawstrings.
Work wise, at most, is two drafts and two gradings. You can draft this whole pattern in about 15 minutes with little to no experience...
by hand. Make two, like you're supposed to for the size range, you've now got 30 minutes invested...and can test the mock up for fit.
So you make the mock ups, and you can make PJ pants in an hour for this, even being a slow sewer (more experience, industrial machines this can be dropped to 15 minutes a pair, especially if you take short cuts and sew the elastic on the waist band rather then a casing). You don't have to make them nice, just test the fit, so you're not ironing or finishing...pinning the hem up to the hem length tests that. And you'll probably use the standard RTW 29.5" inseam plus whatever you want the hem to be, usually doubled with an additional 1.5-2" hem (although for mock up, you're not double folding it, just pinning the hem out) tests the hem length. Time invested in fit is now, at most, 2 hours. Assuming you have to tweak, you then double this time...tweak the draft, resew. Also assuming you're making 2 for grading purposes and not grading just one pattern.
Because of the fact this style of pattern is incredibly easy, it is also one of the first pants you learn to grade...and you can have a working nested pattern in all the sizes in 4 hours as a beginner. This is assuming you're doing it by hand...a CAD program makes all of the time drop dramatically.
So time invested in the pattern? 6 and a half hours doing it all by hand. Even if you make a mock up of each size (which is where pattern testers come in, but if you do it yourself add another 8 hours for sewing (and assuming you sew really slow) you got two or three days of work for someone who has never done this at all. It drops dramatically with CAD and sewing experience.
But, let's look at the pattern. All sizes in one, so we can assume there is ONE draft and grading is done from that. Probably the size 10, then graded 5 sizes either way. 30 minute draft, 30 minute mock up, hour for fitting with the fit model, then a few hours for grading (by hand, not CAD) you have about 6 hours in the pattern. Hold off on releasing so you can get pattern testers (and PDF means there is NO expense to your company) another day for tweaks, another round of testing and you're working on the packaging, contract for printing and merchandising while the testing is going on...so the business has a few days invested in the pattern. Cost wise, they likely have under $250 in the whole shebang.
A highly qualified (20 years of experience or more) pattern maker in Los Angeles or New York will cost about $75 an hour. Experienced professionals in second or third tier cities will charge $35-$60 an hour. (Fashion Incubator, and they will charge for the hour.)
While prices can vary quite a bit, the usual range is from $15 to $25 per size for a 1-6 piece pattern. (Fashion Incubator)
A sample is about $75 for this plus cost of fabric, trims and notions(production samples, also FI) (Where I got my $250 tag, this is less in pounds). Size tests then are free.
So yes, I can see where 8 or 12 GBP is a little much. At this price, she only has to sell about 15 to make any profit.
https://sewguide.com/sew-pyjama-pants/ draft your own
https://5outof4.com/product/pajama-pants/ free with low rise/regular, capri, shorts, pants and include a pocket. No need for a separate file.
Editing here: like anything, you are also paying for the brand name. Also drawstring holes are pretty standard in all garments of this type: 3/4" from center seam, 1/2" buttonhole.