Truthfully, even if it did 90,000, that's a pretty solid number. Considering the change in broadcasters and the relatively unknown opponent, Khan was able to sell a decent amount.
Take 90,000, for example. Essentially, numbers-wise, this would be like if Khan was fighting in America, had an opponent that was only kind of known, switched from HBO to SHOWTIME and sold about 550,000. That's pretty damn good.
If it's 150,000, then it's more like they sold over 900,000. These are pretty good numbers.
Those numbers aren't comparable, in terms of boxing PPV's there is no way that a Boxing PPV that sells 90,000 in the UK is like a US PPV that sell's 550,000. 550,000 in the US is a pretty successful number, for example didn't Cotto-Margarito do around 600,000 and that was on HBO PPV. 90,000 is good considering the late change to Primetime, especially as Primetime is a PPV channel that 98% of people don't know about, it is only known about by Boxing/MMA fans.
You can't just take population differences into account when working out these things out. Mayweather-Hatton sold 1.9 Million PPV's in the US, it sold 2.5 Million in the UK, if they would have had a late change over to Primetime then it might have done 1-1.5 Million but this is obviously a guess as there are no comparable examples.