1 BP per force rating, i much cheaper than during play
Sustaining Foci are not a very efficient use of BP. They cost 10 grand (2 BP) per force rating, and 1 BP per force rating to bind. So essentially we are comparing 3 BP per force vs 2 Karma per force to bind after char gen.
Power Foci on the other hand, are notable for their BP to Karma efficiency: 25 grand (5 BP) + 1 BP per force rating versus 8 karma per force rating. Even factoring restricted gear (another 5 BP), you will still always pay less BP than Karma in the end.
So spending BP on power foci is generally a smart move. But you may want to carefully consider how many sustaining foci you really need to start the game with. Probably not more than 1. Hopefully you can loot them off enemy mages, or just buy them - they're not incredibly expensive. Until then you can always have a spirit of man sustain innate spells on you.