A lot of those came from 'customizations' made by those companies and adopted by the vendors. There is no rule of thumb, as every company is different in its needs. If this was say 100K employees at mutliple sites, then once could look at the estimation via lines of code in current systems. If this is a LAN-based applicaiton with few modifications, easy conversion, etc. I believe that your number is not far from the truth.
Most people think that buying an application is similar to buying wordperfect.
An information system especially HR systems are complex animals that require some level of customisation, conversion of data, entry of new data and correction of old data. The systems must be tested often in parallel with existing systems as one cannot stop one system, install and then start with the new, in the same manner as one would change a production line, these are living applications. The success or failure of many system implementations depend on the ability to implement the change. On e of the things that may be positive in this question is that if they see the size of the cost to make the change, they will begin to recognise the size of the task and this will be very useful as the implementation begins and possible as time goes on. In our last installation, they stated one should not worry about the so called soft costs and only concentrate on the software cost.