Let me ask you something:
If you were running a business and you got some parts that cost you $6,000 or more, how much would you charge your customer?
If your answer if $5,000, then you won't have a business for long. You need to make something and $1,500 on a $6,000 product, or 25% markup, really isn't that much at the end of the day. If you buy a coffee from Tim Horton's the markup is nearly 500%.
Oh, and they're offering FREE SHIPPING across all of the USA. Do you know how much it costs to ship something THAT BIG?
Edit to add:
I just found the parts available elsewhere. MSRP on it is $4,600 USD, then shipping is in the $300 USD range, so lets say it's $5,000. They're charging $6,000 USD. If they had to pay $4,600 for them, which I can believe because they're a direct competitor to GM, $1,000 markup isn't really a huge amount on 12 items. That's not even enough to pay half their staff for a month.