We have a new home. We bought a spec home. The one thing about it that we have never liked has been the bathroom vanity in the master bath. Now after living with it for two years I am about ready to pull my hair out. There is NO counter space barely any drawer space (4 drawers but each will hold a brush and no more) We are considering pulling it out and replacing it with a single sink vanity to gain more space on the counter and in drawers. It is a tiled bathroom so there may be some tile work on the floor. This is the work I think we will be looking at

new vanity
new sink
new granite counter
tile work on floor (very minimal)
probably have to move the light fixture to center one over the sink vs the two that are there now
moving plumbing to center (it is set up for two sinks now)

Materials and labor can anyone give me a rough ballpark figure on what to expect?

I was told that it might be less expensive for me to buy a pre-exhisting free standing vanity with counter and sink already in it even though that would require more tile work on the floor.???

The sky is the limit without any specifics on what you want, what materials you want, etc.
You best bet would be to take some measurements and start looking locally and ask a few guys at your local store these questions based on the research you've done.