Will you necessarily need that mincost if you are new/free? I only managed a -1CR Echidna and she served me really well despite barely being upgraded. Nowadays i do not use her since i have better healers, but she served me well with just that -1CR during times when i actually needed her.

And right now at the stage i am at, she would probably seldom be used even if she was maxed out. So i'd say just getting a copy of a bad unit to temporarily have in your team is still very good and does not need much improvement.

As for the 300 aff item deal. This assumes that we get GOOD units in those events. The ones not being incorporated into the team can wait for a lucky day. During this event i have maxed out Bashira in less than a week and am halfway with Liana, so i'd say those "bad" events are plenty of time if you just grind phalanx2 after getting a copy and/or 3 starred every map.