hello Azizah,
unfortunately, they are right. Repair of iPad is unprofitable because, not many technicians have enough knowledge to it. Technical documentation is residual. But, if you a familiar with electronics, you can repair it yourself. It is not a very complicated.
Many of iPad faults is related to falls and power problems (non-genuine cables and chargers). Unfortunately, opening of device is nesesery that is neither easy nor safety. Many screens are damaged on this stage (that is 50$ for opening :-), by the way - that is quite cheap, mostly is 75-100$). In my opinion (based on hundreds of repairs iPads in my career) you have got problem with power consumption. It is required to connect the meter and check the current consumption during start-up. Based on this, appropriate conclusions can be drawn.
Similar faults cost 200 $ plus about 75% for opening and closing the device (if the screen is not cracked. If it is - 200-250 $ extra for a new screen). As you can see, maybe you should consider sell your iPad (for $) and buying a new one (up to 400$).
* regards