Non-standard parts include:

Ultra-slim desktop power supplies -- Usually $70 - $130 refurbished / used. New parts are sometimes not available. PCR does not ever recommend settling for refurbished power supplies.

Motherboards -- Usually $60 - $150 refurbished / used. New parts usually range from $40 to $150 but may not available new. Even when a seller (on Ebay or elsewhere) claims that a "refurbished", "used" or "like new" motherboard being sold has been "tested", it is still risky to buy because motherboards can pass diagnostic tests but fail intermittently under certain stress conditions. CPU (Central Processor Unit) - OK to buy used CPU chips. Generally PCR does not recommend settling for refurbished motherboards.

CPU chips rarely fail.

Some RAM chips - It is OK to buy used RAM chips.

Laptop monitors -- Usually $75 - $150 refurbished or new.  New parts are often not available.

Laptop monitor hinges, shell casing, mouse pad, keyboards, keys, buttons, etc. -- Usually $75 - $150 refurbished or new.  New parts are often not available. Replacing such parts often requires waiting for a duplicate computer to be sold on Ebay as a "not working / for parts only" computer.

Standard parts are generally readily available brand new, are often available locally, and are usually much cheaper than non-standard parts.

Standard parts include:

Hard drives (usually $40 - $60 new)

CMOS battery (about $4 to $7)

Most desktop fans

Video PCI cards

Audio PCI cards

Some RAM chips

Most mid-sized to full-sized tower desktop power supplies (acceptable low quality ones as low as $35, high quality ones about $70)

Laptop power supplies ($30 new)

 

YOUR QUESTION: What are the most common parts that fail? Hard drive and power supply.