The BA283 boards have what they refer to as a preamp stage and an output stage wired to an edge connector. They were designed to be generic amplifier/driver boards depending on how they were connected or interconnected. The 1290 uses two preamp stages and an output stage. 1272 uses one preamp stage and an output stage. You can uses these as building blocks (like neve did) to make the various bits and pieces that make up the console modules. The transformers and switches are the expensive parts. If you want a P&G fader you can almost spend $100. The boards and other components are not that expensive. You'll need a psu too which will most likely consist of a transformer, regulator, diodes, caps, some resistors, wire, etc. Don't forget a case and perhaps some Xlr's. I think you might be able to do a channel for 250 (being optimistic) depending on the number of channels and your chassis.
The good news with DIY is that you can start a project with fairly minimal funds and purchase your bits and pieces over time spreading out your expenses. I would recommend to plan on it costing more than you think and taking longer. That's not to discourage you from diving in to a 1272 or 1290 project. I highly recommend diving in and never looking back.
Cheers,
jb