The current silver melt value for your coin is 32-USD.
The numismatic value for your coin(1899-s) is 40-USD in VF20 condition.
Be sure the coin is not an 1899 without an S because that is a key date and worth much more. Additionally you want to be sure on your grading because the coin could be more valuable that I quoted if it is in a higher grade.
The value of your coin would rise to 55-USD in EF-45 condition and can be even higher in AU condition, 95-USD.
This link shows the value list for morgan silver dollars depending on condition and year.
http://coinauctionshelp.com/Morgan_Dollar_Mintages_Price_Guide.html
This link gives information on grading your coin.
http://coinauctionshelp.com/How_To_Grade_Morgan_Dollars.html
If you are sure that this coin is vf-20 then the price offered is a good deal for you. If perhaps your coin falls into a higher grade then it may be worth more. Be sure to check the detail in the hair and the features on the eagles chest on the reverse.