Slowdive was one the band from the early 90s whose merchandise are hard to find. Well according to this site they officially made 8 designs of t-shirts (see screenshot below). Sure there are lots of bootleg on eBay and else where, but if you want a real deal, they're pretty hard to come by. Hence, the price of the original one is quite high. I must say the designs of the "Souvlaki" era are very simple compared to their earlier merchandise. But if you're a fan, that doesn't really matter does it?
Check out this ultra rare original "Just For A Day" era tees here and here
Check out this ultra rare original "Just For A Day" era tees here and here