The game's price is $4.99 USD on both Steam and itch.io. If you're using a different currency like Euros or Pounds, then the conversion rate and regional price may differ by platform. The game may also be discounted at different times on the two platforms, though right now they should be the same discount for the summer sale.
Unfortunately, Steam does not support android or APK files, so you can only get the game's android version on itch.io. The windows, mac, and linux versions of the game are on both platforms.
When buying the game on itch.io, you'll be prompted to enter the amount you'd like to pay for it, and it'll display which files you will have access to if you pay that amount. You'll get all versions of the game for paying any valid amount, but you will only get the soundtrack if you pay $5.99 or more (currently discounted to $4.49).