I have just been in Iran for a week, so I will try to answer some of your questions.
1. I dont know anything about the sacraffice festival, but the cost of getting from Tehran to Esfahan is around 5 Euros. There is no online booking available for the buses, but one goes every hour daytime from the central bus station in Tehran to Esfahan. However if your flight lands at 8PM it may take u an hour to get through the immigration etc and I dont know what time the last bus of the day goes at.
3. The safest way to get a taxi is to go to the taxi booths. They are generally honest. To get to the Central bus station from the airport costs around 5 Euros. If u are male(I have been told it is not advisable for women, because some have said they were groped by guys sitting beside them) u could ask for a shared taxi and that would make it much cheaper. Tell the guy at the booth u want to share and he will gather more customers. However if u share I dont know if he will drop u exactly at the bus station. U would have to confirm that when u are booking the taxi. This is the price from Mehrabad airport, where most of the international flights land. To get a private taxi from the other international airport to the central bus station would be 15 Euros.
4. I am a vegetarian so did not pay much attention to what eating out costs. Most of the people I met said the Kebabs which were available everywhere are not expensive.
since you will be travelling around Eid I suggest you use an Iranian travelling agency to book bus and train tickets in advance. (Just search online, there are several big ones.)
You should be able to book flights within Iran through http://www.iranair.com/
BTW, the easiest/cheapest way to get to Iran from Turkey is by (gasp) train. It might be slower than the bus, but since it has sleepers it is more comfortable. I will definitely fly into Turkey and then take the train the next time I go to Iran.
U are welcome Burakhan.
Here is some more information u may be interested in.
U can stay in a dorm room in the Amir Kabir hostel in Esfahan for around 3 Euros per night.
U will probably arrive at the Keveh Bus station in Esfahan. Most of the buses arrive at that one.
There is no taxi booth there(at least I did not see one). If there is no Iranian to help u, then go out as far as the street and ask a taxi driver if he will take u to the Amir Kabir hostel on Chahar Bagh Abbasi Street. If he knows where it is show him a 10,000 rial note and ask him if he can take u for this much. He may ask for 20,000. That is also an OK price to pay.
If u can speak any Farsi bus 301 runs from the bus station and along the street where the hostel is. That costs very little. U will however need a ticket which u would buy in advance from a booth. There may be one at the bus station. Try the ones which sell candy and newspapers. Maybe they also sell bus tickets.