With your pre-built panel being almost $700 more than the others and then on top of that still needing to build/buy the probes and power cord that just seems like a large difference in costs for materials.
I would argue that it's actually low. Remember the $15 -> $40-50 plug comparison I made earlier. That's only one part (hundreds are used) that varies in price by 300% so you could technically build one panel that's 3x the price of another. Have it built in a UL certified facility to get the entire panel UL/cUL listed as an assembly (the way BrewMation does) and you can expect the price to go up again by another factor of 2-3.
So to the untrained eye it appears odd as it appears you can buy the same thing for $X, or 5-9 times $X. So it all depends on what your needs are/what your expectations are. You can buy a many cars with 4 wheels and 4 doors that all take you from A to B that vary in price greatly from $15K to 150K. Know your needs, know what you're getting (not always easy to do).
Price of a product does not only reflect cost of materials either. There's more than that goes into any product. There's quality of the labour, knowledge of the staff, there's support, and so forth. Most electronics/electrical stuff will have a bill of materials (the parts that go into making the device) that is only 20-30% of the consumer cost. (For example, it costs Apple just over $200 to build an iPhone, but they sell for more than three times that).
Ask any store questions before you buy (not just pre-sales questions but the sort of support question you may ask after you've spent your hard earned money). It's not uncommon for any retailer to answer pre-sales questions quickly and post sales support questions completely differently (or not at all). For electrical panels I'd ask for pictures of the insides. This is something that should always be shown (IMHO). It shows attention to detail and quality of the work. Ask what certifications/experience they have building/designing electrical stuff too. Where'd they learn? How long have they been doing this? Are they accredited? Are they building in a UL facility?
It seems now that electric brewing is catching on a bit more there are starting to become a lot of vendors who are also offering services at lower costs.
Very much so. That happens in all industries but is very true with electric brewing I posted free build instructions for anyone to follow and many are building/selling regardless of past experience or knowledge in electrical/electronics work. Many have come and gone. It's unfortunate but because of this free information I now get almost daily emails from people asking if I can assist in fixing panels built by others. Something I never realized would happen. (We did a few panel fixes 6-7 years ago when the emails starting coming in but it quickly turned out to be a giant can of worms as we realized that we'd be responsible for the proper functioning of this unit going forwards. So often we found it prudent to redo the wiring entirely which at the end of the day was more work than just building a new panel from scratch).
There's certainly a lot more options today which can help satisfy buyers at all levels. Choice is good!
Good luck with your setup!
Kal
Sorry if I misled! UL facility built panels would be considerably more expensive. 
