Federal and local authorities are investigating widespread irregularities in the sale and mortgaging of homes built by New Century Homes and its founder John G. Berg. Berg denies wrongdoing, but many of the estimated 150 people who purchased homes from his company say they didn't get the bargain they were promised. Many are struggling to make payments that are as much as 30 percent higher than they were told to expect. The homes are factory built and of suspect quality. The Daily News investigation showed that Berg and his representatives allegedly: promised federal tax credits they couldn't deliver, funneled cash gifts to new Century buyers in excess of amounts allowed under FHA and conventional mortgage underwriting rules, circumvented state and local realty transfer taxes by filing deeds that did not reflect accurate sale prices, and priced their houses significantly higher than those in surrounding neighborhoods.