Use the currency function to format amounts of money.

Plain text: $8.20

In TeX: $\var{currency(a,"\$","p")} + \var{currency(b,"£","p")} = \var{currency(a+b,"£","p")}$

For convenience, you can define a custom function which uses your preferred prefix and suffix.

Plain text: £8.20

In TeX: $\var{pounds(a)} + \var{pounds(b)} = \var{pounds(a+b)}$