There are lots of metrics you might want to use but as you say unless you have it automated it can be a real spreadsheet headache. Luckily I am a programmer :O) Unluckily my marketing isn't as good as my programming right now :O(
Ad,Keyword,Click Cost,CTR,Num Clicks,Conversions,Basket Value,Total Cost for ad, total sales value per ad, average visitor value..
If you are anything like me, many times a day manically ;O)
There really needs to be a public api like the google search api, in which case it would be easy to automate end to end. At the moment you can only automated the tracking part which is pretty simple to do. You still need to input click costs etc but can get ad/keyword performance in realtime just using a simple redirect script that writes the search part of the referrer and ad number out. Match that with sales data and ad spend and you get your ROI.
There are products out there that do bid management etc but they must have had some major cash deals with G or are doing some dodgy screenscraping in which case will get booted one day.