Cost of what? Construction, maintenance, or purchase?

Also, where is the transaction occurring?

Here is some data from the building of the largest replica Viking ship yet built:

http://www.vikingtoday.com/articles/2004/2004-building-of-sea-stallion.htm

Soren Nielsen, the builder, reckons that in the Viking era, it would have taken about 10 skilled ship builders and 5 untrained hands, about 6 months to build a large Viking longship. 28,000 man hours would have gone into the construction, and it would have required the lumber from about 14 large oak trees with trunks of about 1 meter diameter. (There would have been other labor besides the 15 ship builders involved in felling the trees and forming them into suitable lumber. Also in weaving and sewing the sail and making the ropes and other essential equipment. 15 ship builders working for 6 months is about 19,000 hours, the other 9,000 hours are for these other tasks.)

So, if we say the cost of each labor hour today was $10.00, we would get a labor cost of $280,000. Add say $100,000 for the cost of the materials, and you get $380,000. If the labor is assumed to be more scarce and skilled, try $15.00/ hr, and guess $520,000.

It's hard to compare ancient costs to modern costs. However, this compares to the average price of a new built home today in America of $343,000 (or the average price of an existing home in America in Jan 2014 of $190,000). Of course, the price of a home includes the price of the land, not just the cost of building the structure. Still the cost of a large Viking warship might equate to the cost of building 2-3 houses (if you already owned the land that you were planning to build houses on).

Whether a Viking ship would have cost a Viking the same as building 2-3 houses is a different question. Modern houses are built fairly efficiently in man-hours compared to Viking houses, but they have a lot of materials and amenities built in (plumbing, electrics, appliances) that Viking houses would not have had.

In general, labor was probably relatively cheaper compared to materials in Viking times than it is today.

Buying a ship for trade must have been a risky investment, considering the price being similar to that of a house.