Clusters generally work well for tasks that can be parallelised—i.e. you can split the task into lots of small ones that can run at the same time. Fortunately, cryptocurrencies are ideal for that, and a mining pool is essentially a really big cluster for mining cryptocurrencies.
Since proof of work essentially boils down to trying many combinations and hoping to find the correct one, all your cluster would need to do is split these combinations between each Pi so that they can work on them separately. Once the tasks have been split, the Pis can work at full speed testing. I don't imagine in this case the overhead for clustering would be at all high, so in this case I would expect ~100 H/s. If there was more overhead and the delegation of tasks was more complex, then you might get less than 100 H/s. But you could never expect to get more than 100 H/s, really, because each Pi can still only work at a certain rate.
The implications of the above are that if you would make a loss with one Pi, you would still make a loss with a cluster of Pis. You can't scale up the cluster and expect to get more efficient in this case as the limiting factor is the hash rate of each Pi.
As is often the case with the Pi, it's not really optimised for any one task very well.
I plugged some extremely rough numbers into this calculator for Ethereum (though you said you weren't interested in it as much, it is still useful for comparison):
and you can expect a cool profit of... -$0.70 per month. (That's a loss—or as the site kindly puts it, a profit ratio of -100%)
Not great, as you can tell; the electricity cost will certainly outweigh any profits if that hash rate quoted is true. You would break even at 751 kH/s... much more than your 100 H/s.
Bear in mind also that your local electricity cost is probably higher than $0.08/kWh (I would expect to pay double that where I live, at least).
Apparently there are some coins like Magi Coin that can yield $0.20 - $0.25 per day per Pi 3, in which case clustering may be profitable.
A fun project, if you're interested in that sort of thing, but realistically don't combine "Pi mining cluster" and "profit" in the same sentence. Even if you did make some profit with Magi Coin or something similar, the payback time is likely measured in years before you start making any real money yourself.