They said - right in the episode that if it was not Elvis doing all the engine work, the fee would be well in excess of 10,000 GBP. That is like $15K US.
Even then, that strikes me as cheap, but the "well in excess" is a lot of wiggle room.
In real life, I bet Hartech paid to have their services displayed on the channel. Their logo was very prominently displayed. That is unusual on that show.
Further, many folks would simply replace the engine with used and move on.
Not sure I am digging the faux Fuchs alloys BTW. The spoiler and the stickers look fine to my taste though.
I wonder if the "buyer" was the one who chose the back dating features? It did seem some odd choices were made. (or not).