BMW 523i and 523ia

i - stands for fuel injection vehicle (gasoline)
d - Diesel vehicle

so you would not see designation like 318di or 520id etc.

s - in some of the older model it either stands for sport or sedan. In North America, a 325is is a 2 door version while 325i is 4 door. But in Europe a 325is is a Sedan and 325i is a coupe.

ci - In order to standardize the lettering designation, c now stands for coupe (2 doors) and i is injection (gasoline) engine. So next time you see a 645ci, you will know is a coupe with a gasoline engine.

iA - used for 4 door automatic vehicle. Not used for coupes.

x - stands for all wheel drive models (325xi, 525xi or older models 325ix)

t - hatchback 3 series (318ti)

T - Touring (station wagon)

The price of BMW523i of 00yr are around $120k-$150k

P.S.: In Europe, the BMW523i is classify as manual ver. 523ia is classify as automatic.
But in Hong Kong, 523i classify as automatic because all BMW in hong kong are just automatic ver. except water goods. Besides, you may see sth like 523iA in the car show, it because the dealer want to clear to describe to all ppl that the car is automatic ver. so that's why they add A behind the i.

523i is suppose to be manual transmission (stick shift) while 523iA is automatic.

For new car, automatic transmission will cost about US$1800 extra (about HK$14000 to $16000).

Since you are asking for an 6 yrs old model which usually will have about 60,000 miles (about 100,000 km) driven. It exceeds the factory warranty and the price varies quite a bit, based on conditions. you will see a huge price difference.