For BM bits forgeries aside, it does pay to shop round. A browse of the enthusiast boards etc will soon show-up the reputable firms.
Eg, a black box & coils for mine were going to cost me the guts of £500 from a BMW dealer, £300 (OEM) from a well regarded marque specialist but in the end, I paid all of £86 pounds because their guy told me to buy the better-spec Bosch coils (from them) for £36 & a box, which was common to many EU cars & bikes of that age, incl BMW, Volvo & Vauxhall, from my local Bosch agent for @£50. Parts provenance was not in question & Bosch of course were the OEM contractor for BMW.
The only difference, apart from the higher output of the Bosch coils (the OEM-equivalent was cheaper still) was the price & that the Bosch coils came finished in a pretty bright blue!
In the run-up to this, when the bike ate several black boxes over a protracted period, I found a range of prices, from £200 for the BM part, £50-60 for Bosch, £30 for a Lucas one from Halfords to £18 for a no-name pattern. In fact, The cheapest one was £3.50 for a used no-name! None were appreciably better or worse than another.
Similarly, suspension - £195 per shock from a BM dealer. My old shocks rebuilt by a 3rd-party ex-BM mechanic with BM parts - £95 the pair. Decent quality SEBACS, £85 a pair & cheapo Hagons, £50.
True enough there are certain more critical things where I'd always use OEM parts & most reputable 3rd-party firms will advise you when. However, the last time I needed to do that, I also had to go to a dealer because special tools were required & getting the gearbox out & shipping it down south was not something I fancied doing. As a result, I ended-up with a bill of
£1.97 for parts & nearly £500 for labour! The dealer then tried to do me on an MOT to get another few hundred out of me. I disputed it strongly & had the "extra work" reduced to £50-odd. Since then, that firm has been stripped of its dealership.