I believe that the salary for non-national players is slightly higher, the rationale being that it takes more money to get a person to move away from their home. Maybe someone can confirm this.
Obviously this isn't true for BB. If I bid 280k for a foreign player and then someone from that country outbids me the price isnt altered. I suppose you meant in RL basketball which I could see as a player like Yao Ming would have to pay for translators/extra staff to help him acclimate to life in the US or like Stephon Marbury who is doing the opposite currently. Are you suggesting that because of this, it makes sense for foreign players to cost a little more in BB??
But in re<ality this won't happen too, or at least not much ;) Ming get paid by value he have on the court, and that would be maybe less as long he lacks with the language etc. - and i am quite sure that he wants to go to america and gets also lot more money then at home because NBA >> CBA. maybe you would have this when, the national and the foreign league are the same strength and a trainer in the foreign leagues wants his dream player from the other one, but in this case even a national agent could get good money for his player because he know Coach X wants him and nobody else.
In Europe lots of league have rules, that you should have X player of their own nationality or out of the EU in the teams(like initially suggested). This give the national or european players extra value, because you could easily get an solid player but if the solid player should come from germany you don't have many choiches to ask and the player/agents know that ;)