begin{filecontents*}{students.csv}
name,grade
Joe Bloggs,2.7
Jane Doe,1.0
John Smith,3.0
end{filecontents*}

documentclass{article}
usepackage{datatool,fp}

% From egreg at https://tex.stackexchange.com/questions/335483/retrieving-substring-of-dtlfetch
newcommand{DTLfetchsave}[5]{%
  edtlgetrowforvalue{#2}{dtlcolumnindex{#2}{#3}}{#4}%
  dtlgetentryfromcurrentrow{dtlcurrentvalue}{dtlcolumnindex{#2}{#5}}%
  let#1dtlcurrentvalue
}

begin{document}
  DTLloaddb{students}{students.csv}
  DTLfetchsavegrade{students}{name}{Jane Doe}{grade}
  FPevaldoublegrade{2*grade}
  doublegrade
end{document}