[miahi]

Depending on hardware, you can get a PicoPSU or similar ATX power supply at ~$50, but they are usually limited to 160-200W - so no fancy quad core desktop CPU or powerful graphics, but 200W is plenty for a normal PC.

Some LED displays also have an external power brick - if that's 12V then you can wire them to your grid. Unfortunately most of them will have higher voltage to reduce wire costs and losses.

If you can replace at least part of the AC power with 12V DC power, you can then buy a lower power (cheaper) inverter for the AC power.