Charles Runels, MD
Here's an article that explains how placebo alone improved premature ejaculation by 40%. The definition of premature ejaculation has been measured in number of thrusts in some studies, in some studies it's measured in minutes. In this study they used the satisfaction of the man.
Viagra alone did not help more than Placebo. Some have wondered if Viagra (by decreasing the refractory period) might help premature ejaculation. It seems not to help.
Viagra and a topical anesthetic had the same effect as a topical anesthetic alone. You might wonder if having a firmer erection would make you more prone to premature ejaculation. It seems if you have trouble with premature ejaculation that this one's not a worry. Go ahead and use the Viagra if you want; the use should not make it more difficult to avoid ejaculation.
So, the cheapest way to treat premature ejaculation also the most effective: topical anesthetic.
More about curing premature ejaculation
Here's a discussion of the social issues involved with premature ejaculation.