What price per GH/s is worth it to purchase right now?

With this calculator http://www.alloscomp.com/bitcoin/calculator using a BTC price of 75 USD and current difficulty one GH/s will mine $53 in a month. How should one project out after that to account for the rise in difficulty? Just half the dollar amount to account for a doubling of diff every month?

$50 month 1, $25 month 2, $12 month 3, $6 month 4, $3 month 5, $1 month 6 ?

Is 1 GH/s only worth about 100 USD right now, or is my admittedly very crude estimation too pessimistic?