1. The Klaf is generally 48 to 56cm in size. The larger the Klaf the larger the letters and generally that means more work. When you write larger it uses more ink and the Klaf is more expensive. It takes longer to write the letter and more time to complete the sefer Torah. The sefer Torah is a bit more heavier as it gets larger. It can be more beautiful being large though smaller is also nice. It depends on the individual taste.
2. The quality of the writing is also a big factor in price and so is reputation. The writing should be consistent. A nice writing is that the letters are spaced well, they are formed cleanly and that they sofer gives it his own added touch. For instance, the final mem instead of the top line being square, he can bend the top corners out. This gives the letter an extra beautiful look to it. To do this does add to the time writing. If a person is paid well for the job then they would be more willing to extend themselves. It is completely not necessary to make the mem in this way but those who value their work might try and do so. The same goes with other letters. Now its a problem that many soferim extend themselves overly much and they might actually put the kosherous of the sefer Torah in jeopardy thereby. This can be a big problem so it can be better not to get someone who is too into his calligraphy. The main thing after all is #3.
3. Not always addressed enough is the level of Fear of Hashem that the sofer has. This truly should be more important then #2 but is usually put thereafter.
4. Another factor is Kavanah. The spiritual energy invested in the writing effects the congregation. The Torah after-all represents Hashem giving us the commandments. This is rarely addressed but important. There is a concept of doing kavanos, having in mind Kabbalistic intentions during writing. Most soferim are not on the level to do this and it is also more important to concentrate on more basic thoughts and intentions. Anything above that is an added gift and blessing.
6. The price varies a lot based on the above. New Sefer Torah's start at $19,000 and go all the way up to 40,000.