What is a 1987 silver eagle ms-70 anacs certified coin worth?

The market value of a MS-70 (perfect) certified coin depends on the company that did the certification. ANACS has lost a lot of it's credibility since it was sold to ICG and is not quite as highly regarded as they use to be. I hate to say this, but check e-Bay for past sales for an idea of value. Or take it to as many coin dealers as you can.