The cost of sea freight completely depends on your terms with the buyer or seller. There are many types of terms you can be with your seller on. The cost of Sea freight increases with the amount of responsibility your seller takes. If the goods are to be delivered to your doorstep, then the cost is supposed to be higher. If the goods are insured, you will have to pay for the insurance too. The total cost is determined before the shipping of goods, most of the good shipping companies are able to help you with that.
Some other variables that affect the cost are fuel prices, currency rates that fluctuate frequently, seasonal demands, and sea shipping capacity, which acts as a commodity. The value rises when the demand is higher than the supply.