i9505 won't turn on, heats up

Hi all,

I have an i9505.

One day I left it charging while off, went to turn it on, but it does not respond.
I have tried different batteries (that were fully charged), different USB chargers (all samsung) and different USB cables (all original Samsung, all work on my S3)

I have tried recovery mode, download mode, and many combinations of holding the button sequence and inserting battery/USB cable.

The phone does not respond at all. No screen activity, no vibration.

If I leave the USB cable connected, the phone heats up a lot, to the left of the microSD port, when looking at the back of the phone.

I was wondering if anyone else has had this issue and was able to resolve it.

I was thinking about buying a USB charge port/circuit off EBay. Would that help?

I think my only other option is to send it to Samsung and get them to repair it.
Does anyone know what they charge for replacing the mainboard? (I know different countries have different rates, I just wanted a general idea of costs)

I'd appreciate some feedback from users who have had this happen to them.

Thanks