There are several different gearknobs listed for the various variants of 147. If it were me I'd go to a Dealership with the chassis number, get a price, then you'll know the price.
Second hand can have its downsides, as trying to remove on old knob can render it useless to refit. Some versions have a 'one way' thread to ensure proper fitting, so as not to loosen during use.
While I agree that some Dealer prices can be more expensive, its unfair to target them as 'rip offs'. Dealer prices are set by the Distributor, as an example the pure profit on one type of 147 gear lever knob is 3.36 Euro. If we did not have the part in stock, it would be ordered overnight, it would then be transported by plane or sea, and road. The costs of which hardly make it worth providing the service in the first place.