if you can swing it, it's a better than decent investment. especially for keeping a stable microclimate around the strings, not only does it help preserve the look of the plate, but you want to slow the rate of oxidation around the strings, especially the base.
the pricey ones I believe have a hard frame or partial to give it shape and keep the cover in place and from resting on the strings by keeping it just above.
making your own can work if you get the pattern/size just right and can figure out a way to give it some rigidity in key spots.
I have wanted for one a while but it's been cost prohibitive so couldn't swing it yet, may see if I can make it happen this christmas.
they can help modulate slightly the volume of the piano if you use good heavy felt (there was a seller offering a cheaper crushed velvet option I have looked at as well, if the weave is loose, there's less volume dampening).