Its a balance of costs
Not just Mr manufacturer ripping us off lol
First off you have the dev costs
That plastic widget didn't just appear out of nowhere
Then you have the mould tools - Big money - anywhere from £5000 to £500,000, and they don't last forever
Then you need an injection press, and materials, and someone to operate it
Once you have the part produced boxed and ready to sell...
To distributors or retail shops

Now the costs start
Mr Manufacturer, has to have a price that covers his costs to dev and produce the part, with some profit in it for him
However he has to balance over how long that part will be in production for vs how many he thinks will sell
To cheap and he's running at a loss - to expensive and he cant sell them
So the distrubiter takes them and adds his cut - he has to supply to the retail shops
Wages nad transport costs come in here
The shop gets them and adds it's cut
Stocking , premises , advertising, wages & postage costs ( and some profit)
and there you are a part that costs pennies , now costs pounds
Rough rule of thumb , thirds
One third of the retail price is profit, one third costs (wages transport etc) and the last third is the price they buy it at
Thirds is conservative BTW some retailers run at 50%