These are the prizes given to you for collecting mini medals.

Item Number Number of Medals Needed Item Received
001 4 Thief's Key
002 8 Mercury Bandana
003 13 Bunny Suit
004 18 Jolly Roger Jumper
005 25 Transparent Tights
006 32 Miracle Sword
007 40 Sacred Armour
008 50 Meteorite Bracer
009 62 Rusty Helmet
010 80 Dragon Robe And Captain Pose

After you get the item for 80 Mini Medals he will start selling you items:

Prayer Ring3 medals
Elfin Elixir5 medals
Saint's Ashes8 medals
Reset Stone10 medals
Orichalcum15 medals
Pixie Boots20 medals