For $100 it better be a spectacular mantis. 30 lbs of live rock is about $100-$120 if it's free of algae and aiptasia and of good quality. Significantly less if it's got pests or doesn't look like premium pieces.
I would agree with Gobytron - $300 all in or say $250 without the shrimp and it would sell reasonably quickly. You can try holding out for more, depends on how quickly you want to get rid of it
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