1. Tile floor require a very rigid subfloor min. of 1 1/4" thick. The top layer should be a tile backer board, cement board or hardibacker. This often raises the floor far enough to create toe knockers into adjacent rooms, stepped/sloped thresholds can fix this.
2. Costs vary considerably across the country and depending on the job, sorry, check local contractors.
3. There are some good DIY tile books in big box libraries, pick one up. It will answer more questions that we can in this space.