Second question: Yes perhaps by trillions of dollars.
First question: It's complicated. The actual number is hard to measure, since literacy isn't a digital, yes/no property. There is a spectrum of reading ability, and so there are lots of different estimates of the number of illiterate people in the USA. Depending on how you define "illiterate" estimates vary from about 8 million to 130 million.
This is hard to count, by being illiterate you can't interact with the tools that most surveys use. You can't fill out a questionnaire on your reading skills if you can't read!
Nevertheless, by most measures, the level is higher than in comparable countries in Western Europe.
There are multiple reasons: Some are historical. A number of factors created a class of people who didn't read. This has lead to family illiteracy, and a cycle of poverty and illiteracy that continues to this day:
- There is a history of a racially segregated and discriminatory education system. Specifically, black people were not taught to read in some parts of the USA well into the twentieth century.
- There was job market in which reading skills were not needed to obtain a living wage. In the past, there were jobs in farming and factories for illiterate people.
- There is a decentralised education system, with schools in poor areas not getting the same treatment as those in rich areas.
- There are high levels of immigration. Some immigrants may be from less economically developed countries with high levels of illiteracy, or they may read in a different language, but not English.