Every company earns less from each console sold in USA. Each company pays less components, so there's a small compensation. And it's absolutely true that XB360 will suffer less in USA when $ is weak than foreign consoles. But the fact remains that if both things are true, that Arcade profits and PS3 loses, then their cost difference(*) must be greater than $200, this doesn't depend from strong or weak $, it comes simply from their retail price difference. Weak $ means those $200 are worth less, but you know that at least in IT you still buy quite a lot with them, also because components cost is in $ too (for example, you can have an 80GB 2.5" SATA HDD for as low as $32 and a BD reader for as low as $70, RETAIL price).
(*) Including retailers' profit, obviously.