Somewhere, I think a Czech page, someone is offering PLC code for an 8052, if you think its worth it.
That is NOT a PLC as commonly understood. It can not handle ladder logic.

Personally, I wouldn't dream of building one. I use the Crouzet Millennium II controllers, and they are a dream to program and use.
Totally agree, there are real PLCs out there (that understand ladder logic) at $100 up.

Before you even think of designing a PLC find out what the requirements are. Everybody I know can "design a PLC" few have the budget to design a PLC the customer will accept. Someone told me that one of the leading manufactureres spent 18 man years on the ladder logic editor and "compiler".

If what you have to design is a "machine control with a few variables" go ahead, but do not call it a PLC.

Again make darn sure the project is NOT a PLC or you will get burnt. Also, if it not do not call it a PLC or it will be sold as such. The customer base for PLCs have no interst, whatsoever, in something that can not be programmed in their common language (ladder logic). When I was working with machine control I once made the mistake of using a "programmable machine control unit" That machine sat still (and was not paid for) at the customer till a "real" PLC was programmed as a replacement.

Erik