what is the fastest and cheapest way to get to Yi Wu if we are from Xiamen

There is no bullet trains between these two places, so the fastest way is to take the plane.
The cheapest way is the to take the bullet trains to reach Wenzhou first and then take the long distance bus for about three hours to reach your destination. You can take the bullet trains from Xiamen North Train Station and you can book the train ticket on https://www.travelchinaguide.com/china-trains/ and they will deliver the ticket to the place you live.