@boooooob - Does it ship from close location to you, maybe they have warehouses everywhere so shipping on their end is always cheaper too.
Yes, I checked ebay just now, $16-18 with bundled standard gray cable, which is more ideal than ordering a bunch of unbundled Dupont cables. SO I guess that is an OK price, especially with the overnight no wait shipping 
Yes, you’ll have to see if maybe you made a mistake on the other board with FPT flash, or could just be locked differently. Show me image of the error if it persists and I can tell you how to unlock (as mentioned above if it’s Protected Range Register lock)