Terrible in costing

I think I am really bad in own business. The stocks brought back cost me around $1K but it look so little items. I think not more than 40 items. However, the stocks I brought back for KXmama cost $400 extra only, but she have double quantity when compare with mine. how come?

I think my costing is bad enough as I do not know how to save cost and get the cheapest costing. Haii...Didn't I tell you that costing was the worst subject for me in school time?