I've done exactly this for the last few years. I have a bunch of text files with likes like

  # 3rd (of march)
  mail     2.23    mailing for bookmooch
  lunch    6.04    soup and sandwich @ riverfront cafe
  espresso 1.29

  # 4th
  groc    29.57    groc run

etc. I was previously totaling it with a short Python script, but now I'm using a tiny awk script (it's exactly the sort of thing it was designed for). Either way, it's incredibly easy to parse simple text files like that, dump the results out to gnuplot, etc. Just plugging in receipts once every week or two doesn't take very long.