Transfer on Saturdays from Agadir will cost you only 10€ per person (not more than 20€ per 4pax taxi).
There is "Supra" bus station on the main train station of Marrakech, where you can buy ticket to Agadir (10€).
If you fly to Casablanca we can organise transfer for you which will cost 150€
Also you can come by train from Casablanca airport to Marrakech (20€) and than take a bus to Agadir. From Agadir we will organise taxi for you (20€).
Schedule of train. Schedule of bus.