Sum of values spread across tables in document? (like in Word)

\documentclass{article}
\usepackage{array}

\def\q#1{\gdef\thisq{#1}#1}
\def\p#1{\gdef\thisp{#1}#1}
\def\total{0}
\makeatletter
\def\itemcost{%
  \strip@pt\dimexpr\thisp\p@*\thisq\relax
  \xdef\total{\strip@pt\dimexpr\total\p@+\thisp\p@*\thisq\relax}}
\makeatother
\begin{document}

 Bla bla bla

\begin{tabular}{rcrr}
  Quantity & Item     & Price per item & Total price\\
\hline
       \q{5} & Sandwich &           \p{5.00} &  \itemcost
\end{tabular}


 More bla bla bla

\begin{tabular}{rcrr}
  Quantity & Item     & Price per item & Total price\\
\hline
         \q{3} & Coke     &           \p{1.00} &     \itemcost
\end{tabular}

Grand total: \total

\end{document}

---

If you are using tabularx you need to only do the arithmetic on the final run, not when it is doing trial runs to calculate the widths.

\def\q#1{\gdef\thisq{#1}#1}
\def\p#1{\gdef\thisp{#1}#1}
\def\total{0}
\makeatletter
\let\normalwrite\write
\def\itemcost{%
  \strip@pt\dimexpr\thisp\p@*\thisq\relax
  \ifx\write\normalwrite\xdef\total{\strip@pt\dimexpr\total\p@+\thisp\p@*\thisq\relax}\fi}
\makeatother
\usepackage{tabularx}

Here's a solution the takes advantage of some tricks available through pgfmath

\documentclass{article}
\usepackage{pgfmath}
\pgfkeys{/pgf/number format/precision=2,
         /pgf/number format/fixed zerofill=true}
\def\grandtotal{0}
\def\getgrandtotal{\pgfmathroundtozerofill{\grandtotal}\$\pgfmathresult}
%% #1=quantity
%% #2=description
%% #3=price per item
\def\receipt#1#2#3{%%
  \par
  \vspace{1.25\baselineskip}
  \begin{tabular}{cp{1.5in}rr}
  Quantity & Item & Price per item & Total price \\\hline
  #1       & #2   & \$#3             & 
  \pgfmathparse{#1*#3}\pgfmathroundtozerofill{\pgfmathresult}\$\pgfmathresult
  \pgfmathsetmacro{\grandtotal}{\pgfmathresult+\grandtotal}%%
  \xdef\grandtotal{\grandtotal}%%      
  \end{tabular}
  \vspace{1.25\baselineskip}
  \par
}

\usepackage{lipsum}
\pagestyle{empty}
\begin{document}

Some random text.

\receipt{5}{Sandwich}{5.00}

Some more random text.

\receipt{3}{Coke}{1.00}

Grand total: \getgrandtotal

\end{document}

UPDATE

After seeing your comments about Arithmetic overflow, here's a variation on the above pgfmath approach which uses the fp package together with siunitx to make the numbers more readable:

\documentclass{article}
\usepackage{fp}
\usepackage{siunitx}
\def\grandtotal{0}
\def\currentAmount{}
\def\dollars#1{\SI[per-mode=symbol,group-separator={,}]{#1}[\$]{}}
\def\getgrandtotal{\dollars{\grandtotal}}
%% #1=quantity
%% #2=description
%% #3=price per item
\def\receipt#1#2#3{%%
  \par
  \vspace{1.25\baselineskip}
  \begin{tabular}{cp{1.5in}rr}
  Quantity & Item & Price per item & Total price \\\hline
  #1       & #2   & \dollars{#3}   & 
    \FPmul\currentAmount{#1}{#3}%%
    \FPround\currentAmount{\currentAmount}{2}%%
    \dollars{\currentAmount}%%
    \FPadd\grandtotal{\currentAmount}{\grandtotal}%%
    \FPround\grandtotal{\grandtotal}{2}%%
    %% make the `\grandtotal` available outside of environment.
    \xdef\grandtotal{\grandtotal}%%      
  \end{tabular}
  \vspace{1.25\baselineskip}
  \par
}

\begin{document}

Some random text.

\receipt{5}{Sandwich}{5.00}

Some more random text.

\receipt{3}{Coke}{1.00}

Some more random text.

\receipt{300}{Something expensive}{16179.52}

Grand total: \getgrandtotal

\end{document}

You can use the \STsavecell macro of the spreadtab package. This macro permit to use the numerical value of a cell outside of the table.

Update

After seeing the comment of @cgnieder, I add an example based on the provided MWE

\documentclass{article}
\usepackage{spreadtab}

\begin{document}

 Bla bla bla


\begin{spreadtab}[%
  \STsavecell\TotFood{d2}]{{tabular}{cccc}}
  @Quantity & @Item     & @Price per item & @Total price\\
\hline
       5 & @Sandwich &           5 & a2*c2
\end{spreadtab}


 More bla bla bla

\begin{spreadtab}[%
  \STsavecell\TotBevreage{d2}]{{tabular}{cccc}}
  @Quantity & @Item     & @Price per item & @Total price\\
\hline
       3 & @Coke &           1 &  a2*c2
\end{spreadtab}

\FPadd{\foo}{\TotFood}{\TotBevreage}
\FPclip{\result}{\foo}
Grand total: \FPprint{\result}
\end{document}