I think QVC love the high-priced brands - they don't have to stock or sell as many to make a tidy sum. Given that the majority of the YK clothing items are £100+ a pop, they will have a good show even if they only sell 100 of each of 8 per show that's a potential turnover of around £80K if they are then adding in the dreaded P&P... Then of course there will be more than one show on a visit, so you could speculatively do the same calculation per show... it would be great business... Given that some of the other clothing brands are 1/3 or 1/4 the price, you have to carry way more stock and sell more volume per show to make the same sort of money.
I would assume that brands not only pay for their air time they also pay for their warehouse storage space, the higher-priced items work out extremely good value all round. A dress from YK probably takes up no more space than a dress from Tiana B, so the shelf-space would cost the same... so as a % of the price of the shelf-space the Tiana B item would be less profitable overall, especially as they would need more of it as they need to store more stock.
What QVC are very clever with is that they only ever tell you numbers on the big Christmas TSVs like Elemis. Most of the time you don't get any real idea of how many of a TSV or other item they actually have available to sell, so it's tempting to think that in each case the numbers for sale are similar. They won't be.
Of course I could be entirely wrong about how QVC work it all out... and about how many YK items they would sell in a single show... all of the above is purely my own assumptions based on what I understand about business in general.