这工资多少到底谁说了算？

Usually big companies have their own policies in determining the saraly range for a new hire. For small company, however, it will be more flexible, and hiring manager may have more power.

会不会因为我讲价就不给我Offer了呢？

Don't worry too much, they have extended an offer to you, means that they are really interested in hiring you, and salary negotiaion is very normal.

Good luck.