Below is the EXP table for enhancing equipment:
Based on the chart above, you’ll want to only feed 2 or 3* equipment. Usually, people would feed 2* equipment as it can be easily farmed.
As your equipment level increases, it gets more expensive to level them up. One way to save money is to level your 2* equipment to level 5 and then feed it to the weapon of your choice.
A level 5, 2* equipment will give 550 EXP.