We have a legacy classic ASP application that's been around since 2001. It badly needs to be re-written, but it's working fine from an end user perspective.

The reason I feel like a rewrite is necessary is that when we need to update it (which is admittedly not that often) then it takes forever to go through all the spaghetti code and fix problems. Also, adding new features is also a pain since it was architect-ed and coded badly.

I've run cost analysis for them on maintenance but they are willing to spend more for the small maintenance jobs than a rewrite. Any suggestions on convincing them otherwise?