Dylan, you will always have to use up some of the original material used to create the item when you repair it. Otherwise how do you expect to repair something? Even in the real world when you do this to an item like a sword you have to use new metals to fix it.
But yes, the material needed will decrease over time. Nowadays I need around 1 diamond to repair a diamond pick, which used to be around 3 I believe. Tyja is also right, over time your chance to keep enchantments on a tool increases. It starts 0% (Levels 0-99), 10% (100-249), 20% (250-499), 30% (500-749), 40% (750+).
There is a chance that the enchantment will decrease as well. So it would go from something like Fortune 3 to Fortune 2 or 1. The percentages on this are: 100% (0-99), 75% (100-249), 50% (250-499), 25% (500-749), 15% (750+). For this you want the chance to be less, so the higher leveled you are the better it is to keep the enchant you have.
Please note that this is per enchantment. Which means if a tool has three enchantments each enchantment has X% chance of staying, depending on your level. Each enchantment at the same time also has the chance to be decreased, so even if the enchantment stays you could go from Eff 5 all the way down to Eff 1 if you're still unlucky.