You could try the Halfords website to get an idea of what's available.
I searched for back mounted I.e. not towbar mounted for a common car type (Ford Focus hatchback) and of the four options presented, three were Thule, a trusted brand, at not far short of £400, and the other el cheapo at £70.
Would I use a low cost rack for a heavy and expensive ebike? Probably not. It might be necessary to bite the bullet and fit the towbar to open up options.