The current network total hash rate has been around 20,000 Thash/s. You are buying 0.05 Thash/s.
20,000/0.05 = 400,000.
You will get roughly 1/400,000 of the total BTC mined in a day.
Average blocks per hour has been around 8.
8 * 25(BTC/Block) * 24Hours = 0.012 BTC/day
You will need to earn approx. 1.15 BTC to get your money back. If current levels remain this will take around 96 days. The hash rate however has been rising at ~50% a month. You would need to start mining immediately and join a pool, even then you would just break even in a year! In one years time you would be getting less than $5 a month as the hash rate will be very high.
I would recommend taking the $900, buying 1.15 BTC, and use that to buy 23 units of PETA mining shares on Havelock investments. Each unit currently gives 2.88 Gh/s of mining (23 * 2.88 = 66.24 Gh/s) which is better than the 50 Gh/s you are getting. You only get 2/3 back as dividends and the other 1/3 is used to buy more ASIC chips to compensate for the hash rate increase. You benefit from any new ASIC chips purchased, so the 2.88 Gh/s per share will go up as time goes on. The link to Havelock is as follows:
https://www.havelockinvestments.com/order.php?symbol=PETAThe page on this forum:
https://bitcointalksearch.org/topic/havelock-petamine-1150-ths-hash-rate-1ghs-per-unit-310783You of course can sell the PETA shares and get some (if not all) of your initial investment back in a few weeks after collecting weekly dividends.
Personally I think PETA is the better option for your $900. I do own PETA shares and that might make me biased. Keep in mind that PETA is just about to receive their chips in a week or two, so they have not started mining yet. It does not look like there will be delays, but delays could be costly in terms of lost dividends.
Good luck with whatever you choose.