The lack of them coming over is a fairly new development, the Euro leagues have upped their pay considerably as the leagues over there have gained significant ground in popularity, it is something that is just starting to be an issue in these last 2 years.
I may be mistaken but once you sign the Rookie deal, I think you are team bound for 5 years.
http://hoopshype.com/salaries/san_antonio.htm
Look at Mahini's contract above, 2 guaranteed years then team options in year 3 and 4 and a qualifying offer in year 5.
So basically he could be underpaid for 5 years.
That is half of his career.....
DD