Well, if I remember correctly, many of the vikings historically had low maintenance costs. The average viking raider, contrary to popular media, was only armed with little to no armor, a bow with some arrows, and a dagger/knife. Now, I don't expect many people to send out knights to raid unless it's a high value target like a major city much like the Crusaders. Sure, they did "forage" supplies on the way to major cities, but once they got there... oh, boy.