I'd offer Semin 4-4.5 mil a year for 2-3 years with a promise to get him a good center. And would actually go and do that, for Christ sake.
Semin surely is not worth Datsyuk's cap hit (which is his salary right now), but on the other hand, he is second in scoring on the Caps this season and first in assists. Likewise, he last year he had the same ppg as Backstrom, who was also making 6.7 mil a year.
And yeah, Backstrom was injured this season and had a down year before that, but that is the point of having someone who is not Ovechkin or Backstrom and can score and make passes.