I have a Rails 2.3.X app. I plan to hire a developer to either update the site to current Rails standards or rebuild the site. In general, is there a way to evaluate whether it's more cost-effective to update versus rebuild? My preference is to update.
Hard to form a judgement on this, it depends on number of lines, gem dependencies used (more is harder), test coverage (less is harder).
We have tried to upgrade an application of 100.000+ lines from Rails 2 to Rails4. It certainly is a lot of work. Building from scratch is a serious alternative, especially since it is nearly impossible to timebox the upgrade project. We are rebuilding the app. Also, Rails 2.3 is more than 5 years old. I can imagine that your market may have changed and it needs some rework anyway because it is outdated. The last thing I want to mention is tests. If your app has considerable testcoverage, updating may be easier.