That assumes that HS2 will make a profit on its operation, which cannot be assumed, if its like the rest of the rail network it will need an operating subsidy which is likely to be on top of the existing network costs
HS2 would be much more likely to make a profit than the existing TOC's.
Let's take London Manchester as an example:
Currently 3*11 coach trains
Future 3*16 coach trains
So HS2 services will need more coaches, right?
Well no, currently each train takes 2 hours each way with half hour turn around at each end so we need 5 units to run the service at 1tph, or 15 at 3tph.
No with HS2 this chances to 1 for each way with half hour turn arounds so a total of 3 units to run the service at 1tph, or 9 at 3tph.
In terms of coach numbers the current diet would therefore need 165 coaches vs HS2 where it would need 144. Now or may well be that there's currently some 9 coach services, if they were all 9 coach services then HS2 would need to lease just 9 more coaches.
However that's only one cost, so let's look at staff time (which is a function of staff costs).
Again with more capacity it's likely that there'll be a need for more staff, however again the journey time comes into play.
Assuming (simplistically) that each crew run with each train then you'll need 5 currently and 3 for HS2, as such if you increase the number of staff from 3 (driver, guard, first class host, with trolly staff covering their costs) to 5 (Driver, two guards, two first class hosts and trolly staff covering their costs) then your staff costs would be the same.
Only they won't, they would be lower under HS2, as drivers generally get paid more then guards and guards more than first class hosts, so even doubling your customer facing staff you'll still have lower staff costs.
As such so far hardly any of the costs have increased significantly and some like staff would likely fall.
However the thing which makes the big difference is that you've broadly doubled the seating capacity of the trains. As such the potential for extra ticket income is significant, even if you discount some of the tickets, the fact that you could easily sell another 200 means that you could fairly easily be bringing more income.
Now given the Avanti had been paying a premium to the government, HS2 would have to do something significantly wrong to start making a loss.