Due to the design of the HK clones, you should be able to simply pull the pins in the rear cap and remove it, then replace with a standard stock (after receiving your approved stamp from ATF, of course.) I have a V51P pistol that I decided to SBR and in that case, I had to get a very specific stock with a collapsible recoil rod which I was able to order from Vector. You might want to take a look at the HKPro board to see if anyone has posted anything specifically related to your interest.
Pre-built (factory?) SBRs go for more than the DIY ones simply because most people don't understand the (1) ease of construction and/or (2) legal process, IMHO. If a shop is putting it together, presuming they are licensed to do so, there is the cost of the additional parts to put the stock on it and make whatever mods might be required. Turning a 'pistol' into a SBR can be as easy as swapping uppers on an AR or it can entail cutting, threading and re-crowning a barrel on another type of gun. In my case, SBRing an AR cost about $50 (not counting the stamp) in addition to what I had in the pistol. But the stock for the V51 ran about $300, so the cost can vary with design. ARs are very "modular", HKs less so and AKs and FALs almost not at all from what I can see. But I am not a gunsmith by trade and some of the ones I know can knock these things out with seeming ease.