I don't think schools that have gifted programs typically require parents to pay for IQ testing. So what do you think causes underidentification of the non-affluent?
IQ testing isn't usually what is used to id kids as gifted. Group ability tests, achievement, and scales filled out by teachers are the std fare. Affluent parents with gifted kids who underachieve or who are not convergent thinkers who are identified on multiple choice group tests may be able to pay for private IQ testing. Poor families usually are not able to do so and also may just take at face value what the school says -- your kid isn't gifted b/c his group test scores say so.
