Much of that cost is exactly that - dealer markup. There might be some build quality differences as well - depending on who made the gas struts.
What I've found is that you could get the exact struts (brand, make, part number) that the OEM uses for considerably less, but doesnt have the right ends on it. Or you can get an aftermarket one with the ends, but be a slightly different part. Of course the ends are not easily interchangeable.
Also have to think about online shops - they generally dont have to stock any of these parts - most have a list of vendors that they deal with and the vendors will drop ship that produce to you - hence the savings is past onto the consumer.