How to compare old road bikes
A reasonable estimate can be found using a site like Bicycle Blue Book. This site is a database of used bike sale transactions going back for almost a decade, and has a number of pretty obscure manufacturers listed. You can even add upgrades you have made from the stock and get an estimate of how that changes the value, though it doesn't account for things like racks, fenders and lighting systems you would add to a bike to make it a competent commuter bike.

I've sold 4 bikes within the last 6 months, and all 4 sold within a reasonable range of what was listed on that site.