When companies start to program an internet shop for customised products, the costs are usually underestimated at the beginning. The cost situation can best be made clear by the image of an iceberg. At the beginning you only see 20% of the costs. Only after going deeply into the matter, it comes apparent that considerably higher expenses are necessary.
Software applications have nowadays a life cycle of 1-3 years. The technology is changing so fast in this field, that after 2-3 years most applications become obsolete, if they are not updated.
In the ASP model the software provider usually steadily updates his software thus prolonguing its life cycle.