After you complete the first world, you'll gain the ability to spend coin on toys on the Astroship.
There are total of 107 toys. So you'll need 53 (they rounded down) to get this trophy.
Fire (16) + Sky (21) + Urban (16) + Cave (16) + Water (21) + Forest (16) + Space (1) = 107.
Each toy cost 100 coins, so theoretically you'll need 5300 in order to get the half of the toys. However, if you grab a black bomb ball, it'd be a hundo dinero wasted. I did, however, manage to accidentally grab two toys at once -- so luck can go either way.
You must complete about half of the levels in order to unlock half the toys, (somewhere between world 2, level 3 and world 3, level 1) regardless of how many coins you've collected before then. If you manage to get ahead of that curve and get too many toys, the crank machine will boot you off until you've progressed further in the game.
Pretty straight forward, should be doable with normal game progression. Happy trophy-gettin'.