This is 66% off the standard price of $39.95 including at Bunnings. Average online price appears to be about $19 on sale. This is the X-Lock version which is usually more expensive than the standard universal version.
Standard for Universal diamond cutting disc - for reliable cutting in general building materials
Particularly suitable for applications in pumice stone, curbs, screed stone, limestone, clinker brick, mortar, aerated concrete, sandstone and brick
Also suitable for concrete, roof tiles, limestone plaster, marble, fireclay bricks, slate bricks, composite bricks and cement bonded fibre boards
X-LOCK accessories are compatible with angle grinders with X-LOCK mounting, as well as with angle grinders without X-LOCK mounting of all brands
Diameter: 125 mm, bore diameter: 22.23 mm, cutting width: 2 mm, segment height: 10 mm
for reliable cutting in general building materials, skin and bone.
Good find OP, that's a great price.