It's rather hard to produce such a share even with Radeon HD6950 at 448 kH/s.
I had to set the difficulty modificator (after my WDC address) to "+0.00196000", it provides "the second part of share difficulty" = 128 and "the first part of share difficulty" at least 128. It allows to produce shares like 30K/128 (and higher) nearly once per hour. One such a difficult share is enough to get several payments. The more such shares you produce, the greater your payments are.
People having rigs of several HD7950/7970 can produce more shares and more chances for them to be so difficult.