Yes, here’s proof that you really can get a roof carrier for under £20! For a seemingly simple rack, this required quite a lot of assembly work, and corners have clearly been cut in design and manufacture. For example, the large, plastic-covered nuts for securing the rack to the roof bars interfere with the tyres when the bike is mounted, and will also collect water.
Elsewhere, the supporting arm pivot consists of two thin pieces of flattened steel tube, a washer and large rivet, and is quite flexible. Up top, the hard plastic clamp is quite small and only held the sides of an averagely fat down-tube, though its stated limit is 45mm.