I probably got by VERY cheap compared to most for an equivalent design. Why? I already had the kettles. But the key thing is shopping around and design flexibility.
Vendors:
1. Ebay (mostly Chinese vendors), and
2. Amazon
3. local HW store
4. BrewHardware and/or BargainFittings type of place
You gotta be careful AND patient with #1. I bought two PID controllers with manual mode (a lot of research) and SSRs. They are not as user friendly as the Auberin ones, but they were alot cheaper and work just fine. The RTDs, most connectors, switches, and indicator lights are all shipped from China. Inexpensive, good quality, but took a month.
I kept an eye on Amazon (#2) prices for plugs and receptacles and some cable, and found better prices than ebay on these. I even made a few design decision changes based upon what I could get cheap.
#3 - What can you say? No matter what you do, you may find some things at your local Homedepot, Lowes, Ace Hardware, etc. that, because of shipping and/or convienience, are the ticket. Generally, though, they are limited in terms of what you need, unless it could be used in homebuilding. (120V receptacles, wire, etc.)
#4 - Stainless parts are not cheap, but at least these two vendors have what you need to convert your kettles/pots for electric brewing without welding (e.g. - weldless fittings, sight gauges, etc. in stainless). I kept thinking the Chinese ebay vendors or Amazon could be cheaper for these, but it wasn't so.
Don't neglect shipping costs. Most things you need cannot be bought locally, and there are many vendors you must buy from. It's sometimes better to pay a few bucks more from a vendor where you are already buying things to save on shipping costs. And vendors with free shipping are essentially discounting the part(s) significantly.
Going cheap is a matter of having the time and being OK with less than spectacular bling. You can follow some build like Kal has done and get an outstanding build with much less effort and research time, or you can play with the numbers and be patient and do a lot of research.
Finally, consider BIAB electric. A single vessel design can save you a bundle. One PID, one SSR, one kettle, one heating element.