Guild Wars 2

Im thinking of leveling my crafting skills the easy way by only crafting the same item over and over again so that i will focus on buying only that ingredients.

Question is will the experience gained on that item be the same until i reach level 400?

No. Experience decreases as you out level the item. Eventually it will give no experience.

This is not a good way to level your crafting. Do cheap discoveries.

Alchemy potions may be an exception, but check in that post below.