Cyrus is probably never worth it, you can get him from premium summon even. If you want to CR/SU spica then go ahead i guess, otherwise Saki and Akane are decent things to buy. Saki will probably be quite useful on a few maps since she's essentially a melee slot archer.

That said, you can always wait for their next trading post announcement and see if there's anything from that which you want.

As a completitionist i bought all 3 :P