80-20 Rule

Most customers like this because they get an awful lot up-front for not a great deal of outlay.

However, the problem comes when they want to implement some of the additional functionality that was too expensive the first time round. You see, mentally the customer has spent (say) 20K and got the majority of their system in place. At this point they forget the 80/20 rule and start to imagine that because the system cost 20K the final 20% of functionality will only cost about another 4K - or at worst, maybe 10K.

Oh dear, the corollary of the 80/20 rule (80% of the functionality for 20% of the price) is that the final 20% of the functionality ends up costing four times the original price.

Perhaps a slide show will help illustrate this: