The Fenris has zero strike capablity. An LTerSlash gets beat out by a bank of Tempests.
By FS2, I think the Fenris is on the way out; you only see one in GTVA colors once in the main campaign, IIRC. If nothing else, their sheer fragility in the FS2 era gives them a life expectancy of, on average, a minute once they're seriously attacked. Even the Aten lasts longer then that. (Though not by much...)
Concerning the Aeolus, cost-effectiveness is...beating 8+ wings of bombers vs. 1 wing of bombers for the Fenris, 2 wings at the outside. Unless the Aeolus costs more then four times what the Fenris does (unlikely; the Fenris is actually larger, so at the very least it needs more materials and probably more crew), the Aeolus is coming out ahead. And this isn't even considering their respective anticapital abilities. (Fenris: laughable vs. Aeolus: not great, but serviceable.)
Where cost and the Aeolus are considered, there is almost zero possiblity that it's not cost-effective when compared to the other cruisers. Rather, if it's not cost-effective, it must be in comparison to the Deimos.