One advantage of the "Pub Prop" over a home-made version is that it comes with adaptors that suit smaller-necked instruments such as mandolin and fiddle, and also has a built-in bow holder. Perhaps most importantly (from the point of view of whoever's table you are clamping it to) the clamp is very well cushioned and will not mark or bite into the surface to which it's attached.

At just under £12 (a little less than $18US), I wouldn't describe it as "pricey"; good value for what you get, I'd say.