They have each of these cards appearing in at least 3 different listings with various prices. These are the lowest.
32GB
$10.36 - https://www.ebay.com.au/itm/232958583846
64GB
$17.18 - https://www.ebay.com.au/itm/230742335092
128GB
$29.96 - https://www.ebay.com.au/itm/232958583846
$26.36 - https://www.ebay.com.au/itm/233011496902 (eBay Plus only)
256GB
$73.57 - https://www.ebay.com.au/itm/372052530931
$73.57 - https://www.ebay.com.au/itm/232958583846
(for eBay Plus members there is a better 256GB deal here: https://www.ozbargain.com.au/node/422367)
Question from someone who knows not much. Are these what I would use to increase storage for a Samsung galaxy S9 64gb? Thank you