I was involved in the conversion of over 500k lines of Visual Basic 6 to C# motivated by Microsoft's EOL'ing of VB6. Most of it was a desktop application although part of it was a server component. We used an automated code conversion tool which did a surprisingly good job of handling the mindless parts. Some pieces such as the database interaction code had to be hand-ported. I was pretty surprised that we were able to pull the whole thing off while maintaining functional parity. In the end, there wasn't much of a change in performance, but C# proved to be a much more productive development platform than VB6 in terms of tooling, ability to refactor, use automated test tools, etc., so the team's velocity increased significantly over the next few years.
We used a code conversion tool from Artinsoft (now Mobilize.Net) and brought one of their consultants onsite for a week. The cost for that was in the low 5 figures, but our CTO (rightfully) recognized that as a bargain given the amount of developer time it would save us. The consultant helped us analyze the code conversion results and got us some custom builds of their tool to handle certain edge cases in our code. We had to make a pass over the converted code by hand for certain things such as changing from VB6 to C# coding conventions, etc., but as I recall, the overall effort was 3-4 months for a team of about 10 developers. That's not bad considering it was a critical part of our flagship product in ~2010 and MS had dropped support for the VB6 IDE.
