Figure out how much fiat you laid out for those 3 Neptunes, divide that total by $550 (or even $420 if you are a quick trader and caught the bottom). That's how much BTC you should be able to mine to come out ahead. If your 3 Neptunes mine more you have profit. If they mine less you have loss. Simple as that. Difficulty modeling suggests you will come out losing, but nobody can say with certainty.
Basically, I agree with you that BTC mining is likely going to burn money (the exponential growing hash rate makes winning this game very unlikely).
However, I recently started to take an alternative path to mine BTC that works extremely well, so far. Instead of directly going the sha256 mining route I invested in scrypt mining. Well, at the end of the day I am only interested in BTC. Therefore, I use a multi-pool that mines different types of altcoin and automatically converts them back to BTC (for example: middlecoin and clevermining). The advantage of altcoins is that the hashrate does not expand like crazy, it seems the hashrate is much closer to the equilibrium, e.g., see
http://bitcoinwisdom.com/litecoin/difficulty .
My thought is as follows:
http://clevermining.com/profitsFor the last months clevermining returned about 0.011 BTC on an average day per MH/s. I rented 25 MH/s for ca 28 BTC and will likely get a bit more than 7 BTC this month, which is a good value. If the hashrate goes a bit up I am still fine (but for the last three months it was pretty stable). As long as the hashrate does not increase like in Dezember (which is unlikely as GPUs are hard to get these days) I am fine. And even if this happens, this likely means that BTC / LTC prises soared and in this case I am happy for my saved BTC.
First I built a mining rig on my own, but later I found that "genesis mining" is cheaper than building my own rig (and no noise and heat at home).