Usually $1.25 is said as "A dollar and twenty-five cents." It can also be said as "A dollar and a quarter." But note that just because .10 (ten cents) is a dime, it's not idiomatic to say "A dollar and a dime." It would seem awkward and nonnative to a native speaker. We also say, "A dollar twenty-five."
For the larger figure, 1.2 billion, we say, "One point two billion." This is the case even if it's a dollar figure, e. g. $1.2 billion.
Note that when we're dictating, we might use the term 'period' for the dot; that's mainly to be understood. But for websites, we say dot, e. g. "Ebay dot com."