Hill climbing with noisy data
When in doubt, bootstrap.


Make a list with 970 0s (the non-sales at \$10) and 30 10s (the sales at \$10) - customers_10
Make a list with 975 0s (the non-sales at \$20) and 25 20s (the sales at \$20) - customers_20
Repeat lots of times (maybe N = 100,000 times - increase this until the the result settles down)


Randomly take 1000 samples (with replacement) from both lists - samples_10, samples_20
Sum the samples for each list - sum_s10, sum_s20
Record which one generated more revenue (i.e. whether sum_20 &gt; sum_10)

The percentage of times sum_s20 was greater than sum_s10 is your X


(Another approach, which would be equivalent in the limit of N large, would be to model this as a binomial process. But processing is cheap, bootstrapping is robust and easy to implement - getting the binomial model right would be easier to mess up).