An 1899 $1 "Black Eagle" Silver Certificate graded PMG Very Fine 20 (VF 20) is generally worth between £170 and £290.
The exact value fluctuates based on the specific Friedberg (Fr.) catalog number, which is determined by the signature combination of the Treasury officials on the bill.
These notes are Very Rare and difficult to find in Very Fine condition, like this one.
With the Teehee/Burke signatures, this note typically retails for ~£200 on average.
'Tis a lovely $1 bill, and over 125 years old!