After trying out a bunch of checkbook apps that were both pricy and had too many features compared to what I needed, I decided that I could do it in TaskPaper.
To use this. Just make tasks with the format "Entry name: Amount" (like "Groceries: -30.80"). Then select the tasks you want to total and run this script:
on splitText(delimiter, someText) set prevTIDs to AppleScript's text item delimiters set AppleScript's text item delimiters to delimiter set output to text items of someText set AppleScript's text item delimiters to prevTIDs return output end splitText tell application "TaskPaper" set selected_entries to get selected entries set total to 0 repeat with each_task in selected_entries tell each_task if entry type of each_task is task type then set theContent to text content of each_task set thelist to my splitText(":", theContent) set amount to item 2 of thelist set total to total + amount end if end tell end repeat end tell display dialog total
The script adds up all of the amounts that you have highlighted (it ignores project and note lines) and displays a box with the total.