A while back I have bought a cheap second hand Commodore 64. Unfortunately it was broken, next to the PLA chip which I have fixed by buinding a custom CPLD version of it, the BASIC ROM chip (901226-01) was also broken. At the moment it is only possible to buy one that is salveged and the ones that I found were around 8 euros. A cheap alternative is to burn your own EPROM version of it.
The most important chip in commodore 64 computer is the PLA chip. When this chip is broken, nothing will work and most certainly you will get a black screen after you try to turn it on. The original replacement chip is hard to get and if you find one it is not cheap. Luckily there are alternatives for replacement. It is possible to create one using EPROM, however the results of it's performance vary, or you can buy one on the market like SuperPLA, realPLA, U17 PLAnkton or PLAtinum.