Why do DC motors cost more than AC motors of same capacity?
Motor physical size and manufacturing cost is more closely proportional to torque capacity than to power capacity. If you compare two motors with the same power capacity, a higher speed, lower torque motor will be smaller and cost less than a lower speed, higher torque motor.

A motor with a higher operating voltage and lower current requirement will have a lower cost than a motor with lower operating voltage and higher current.

A 1000 kW AC motor will most likely be a three-phase, squirrel-cage, induction motor. Since the current is carried by three conductors rather than two, that represents a slight advantage over the DC motor.

The wound rotor and commutator of a DC motor is quite a bit more complicated then the rotor of an induction motor. It is made out of copper and iron rather than aluminum and iron. The heavier rotor of the DC motor may require more expensive bearings. The rotor design is probably the most important factor in making the DC motor more expensive.

If the DC motor stator has permanent magnets, that could make it more expensive or not depending on the magnet material.

There are many more manufacturers making AC motors and many more AC motors used at 1000 kW. At 1000 kW, DC motors have rarely been used except in applications requiring adjustable speed. That gives the AC motors a big advantage in manufacturing cost because of economies of scale. More competitors making AC motors would tend to drive the selling price down.

Since DC motor buyers are buying motors for adjustable speed use, they are buying an electronic speed control along with the motor. They will be comparing the motor - controller package prices, not the motor prices. They may also be more concerned about quality than a lot of AC motor buyers. The least expensive AC motor on the market may be of lower quality than available DC motors rated 1000 kW.

Regarding Motor Power and Type and VFD Use

In the motor manufacturing industry, a large motor generally refers to a motor that is physically larger than the frame sizes with dimensions specified by IEC and NEMA. In terms of power, large motors are generally those rated about 400 kW and above. Medium motors are rated about 1 to 400 kW. Small motors are 1 kW and less.

Induction motors are available with power ratings up to about 22 MW, but there are more manufacturers at lower power ratings. Induction motors rated a 1 MW are not at all uncommon. Large induction motors are manufactured by Siemens, TECO-Westinghouse, ABB, General Electric, Toshiba, WEG, Marathon Electric and others.

Large synchronous motors are manufactured by Siemens, TECO-Westinghouse, ABB, General Electric and Toshiba and probably others with power ratings between 1 and 100 MW.

A synchronous motor might be selected for a 1 MW application, but an induction motor seems more likely. In either case, the operation voltage is not likely to be less than 2000 volts and more likely to be about 4000 volts.

It is really not likely that a DC motor would be considered for any application at the 1 MW power level. If it is considered it would not make sense to consider only the price to the motors. A reasonable comparison should include the motor, controller (or starter) and projected future maintenance costs. Even including a VFD with the AC motor, the DC system is likely to be more expensive, particularly if maintenance and reliability is considered.

VFDs are available for both induction and synchronous motors at all power levels at which motors are available. Most large AC motors are not used with VFDs, but many are and the very largest motors may require VFDs for starting.