So, most big items, such as Infinity Edge, and Guardian Angel are made by purchasing smaller items and upgrading them into larger items.
With the Guardian Angel, for example, it is built out of:
Null-Magic Mantle (400g)
Cloth Armor (300g)
Chain Vest (700g)
And then it costs another 1200g once you have all of these in your inventory to combine them into the Guardian Angel.
The total cost to make a Guardian Angel is 2600g.
But, you don't have to actually purchase any of the first items to get a Guardian Angel.
You can buy the Guardian Angel flat out if you have 2600 gold.
Or you could buy all 3 items then pay 1200 gold to combine them into a guardian angel.
Or you could buy say the Chain Vest for 700 gold, the Null-Magic Mantle for 400 gold. Now you have the Cloth Armor (300g) and the combine cost (1200g) left. Instead of buying the cloth armor, you can just wait till you have 1500g and combine the chain vest and null-magic mantle into a GA.