It is impossible to get paid for a difficulty 1 share but shares doesn't have to be difficulty 1. Why couldn't one use a higher difficulty share?
Revenue per difficulty 1 share: 25 BTC / 21,335,329.114 * 1E8 = 117 S ea (at current difficulty)
Dust Threshold: 5430 S
Maximum # of independently payable shares per BTC of block reward: 1E8 / 5430 = 18,416
Min pure PPS share difficulty: (block difficulty) / (18,416 * current block subsidy)
Min pure PPS share difficulty (at current difficulty & 25 BTC block): ( 21,335,329.114) / (18,416 * 25 ) = 46 diff.
Value of difficulty 46 share = 25 * 46 / 21,335,329.114 * 1E8 = 5390 S ea
At difficulty 47 a 1 GH/s miner will find ~ 17 shares per hour (average time between share 201 seconds)
For simplicity I would round to nearest 10 difficulty and change every difficulty adjustment. Too easy. The last 8 difficulty periods would be
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04/05/2013 20 diff 6516 S
04/17/2013 20 diff 5571 S
04/29/2013 30 diff 4962 S
05/12/2013 30 diff 4469 S
05/25/2013 30 diff 4114 S
06/05/2013 40 diff 3203 S
06/16/2013 50 diff 2585 S
06/29/2013 50 diff 2343 S