That's a practical situation, good that you're focussed on your favourite characters. Guess it is much easier to be a completist in one/few lines. Else, one ends up getting confused and/or going overboard.
I wanted to get Cover Girls; well, at least the Cat, Harley, Zatanna, and Ivy. But somehow missed on them when they were going for $50-60. Now the prices are crazy! However, the way how sucessful this line went, hope DCD will rerelease them, just like what they're doing with the B&Ws.
Aliens and Harley, hmm, quite an interesting contrast I'd say.
Yeah, collecting one character from each line is pretty simple and way cheaper than trying to collect every character from every line out there. My GF would kill me if I did that with this DC line. I did that whole "buy everything from that line" with the Alien line and my bank account is still hurting till this day!
Anyways, I was also tempted to pick up the Cat and Ivy to make the Sirens, but like you, I was deterred from picking them up from those crazy prices sellers were asking for on eBay! Oh well money saved.
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Back on topic..
I just pre-ordered AC Harley this morning from Urban Collector. Their discount wasn't as great as other online venders but, their cheaper shipping quotes made up for that.
Hope she looks as good as the prototype!