Mower tyres are very slow moving stock , bulky and go off in storage
That makes them expensive to supply as replacement parts
Being made in short production runs makes them even more expensive .
So those prices are quite reasonable
If you change to a different brand let alone size then both must be replaced or you throw the mower out of level .
ATV & Quad bike tyres are usually less expensive as there are fewer sizes so they get made in larger production runs and they are replaced a lot more often then mower tyres.
Shops that sell a lot of off road tyres will also have lower prices
Down side is ATV tyres have more aggressive threads so require a lot more care when turning to avoid ripping up the grass .