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.