The cheapest 18ct gold is the Portofino automatic. Then by the time you are looking at the gold Portuguese perpetual calendar you are looking at a watch approximately five times the price of the Portofino, but we have not finished by any means as we could take the latter and times 10.
Prices vary according to the Market you are in and the differential will vary possibly country to country.
Be warned there are a load of second hand watches available, some are excellent and others are marriages of a movement from one watch, a dial from another, a case from third. How many of those components are genuine IWC is one question but the main problem is the fact that if they are a marriage they might not be serviced by IWC as what is the watch? Also at the worst end of the scale some watches may be anything from a modern case with old movement right up to total utter fake! If your knowledge is minimal you are entering a minefield which could cost you decent money for a watch with a Market value of scrap gold.
By the way this forum is non commercial and prices also are discouraged. Can not even recommend certain auction houses. But safest place is from authorised dealer, but their stock will be new 99% and very few trade ins available!