I paid around $1,800 for mine brand new. My 06 Sierra is Greystone Metallic with a 5'8" bed. That being said, I could not find one in the size and color for a real fit.
My 05 Silverado was white with a 6' bed, and I paid about $700 for the same brand and style (Leer). The GMC one looks new, has a removable window for cleaning at the cab, and warrantied. The Silverado one had some chips on the edges and did not have a removable window. One of the sliders was also seized in place. I also had to RTV the 3rd brake light into place as it was falling out.
Depending how nice you want the truck to look, and how picky you may be on options, attention to detail, etc I would say ordering one is a great idea, even more so with a specific color. If you have a common color and size, search around on Craigslist, FB Marketplace, and Offer Up.
I love my Leer; pop out glass by the cab, perfect color match, slider windows with screens, carpeted inside, and metal frame back window with locks.
The used one did the job, the new one has been perfect.