If you compare it to TR, then yes it looks overpriced.
It doesnt necessarily mean that the cost has to be decreased. Not every tech in the game sgould have the same value for its price (just like bloodlines vs hussar). Abs the good thing about UT is that you can make them specificaly overpriced or underpriced to balance the civ. Vikings lost TR for balance purpose, to mitigate their strength in mid castle age to early imperial age.
Yes, because it is very expensive upon imperial, since you already take bracers and chemistry before it. If they do nit tech switch out of arbalesters, they should still research it a few minutes later, as soon as they can afford it.
For whatever reason, it has been decided that the archers play and knights play should have the same strength on the map & setting where archers are at their strongest (1v1 Arabia)
Bosveigar should be researched even later than that, also as soon ad you can afford it.
Switching to berzerks may be too expensive and time consuming, as you may not have researched any infantry armor upgrade. You can add some skirms , but you should usually not switch to it because skirms are bad against every other unit that the opponent may switch to, while your arbalester push should keep the momentum and threaten any army.
You can add rams or mangonels as well. But if your arbalesters are able to pierce through skirms, then I would rather stay on arbalesters than switching to skirms or berzerkers, as long as I have gold (map control).
If I am on the back foot, then against skirms I would not stay on arbalesters and research bosveigar.
Why ? Are you afraid of lacking gold in the late game ? Would 600.f 1000w be better for you than 600.f 500.g ? I suppose that the high cost has some balance purpose, while taking into account Vikings’ top tier economy.
As said, for me the actual price of UTs/UUs do not need to align with other techs/units,  and should rather be set to balance the civ according to win rates and play rates.
And apparently, Vikings were too strong as a fast imperial into arbalesters play, and they lost TR to mitigate that. The Bosveigar price + castle requirement represents the amount if time the TR equivalent tech should be delayed for Vikings to be on a satisfying spot balance wise.