The price per model of an upgrade like the Eviscerator is not the same as the price for a unit. It can't be, it wouldn't make any sense. Otherwise, Terminators would be like 100pts per model. Repentia aren't 40+ pts per model because, despite Eviscerators typically being 25pts, a fairly fragile unit of repentia would never be able to deliver those Eviscerators into combat where they could actually do anything. Therefore, 10 repentia isn't a 400pt unit.

When you're looking at the effectiveness of a unit, you have to consider it's overall effectiveness vs cost. You can't think "oh, Eviscerators are normally 25pts each, so each repentia should be worth 30pts". That would make them horribly overpriced.