>>>Not just the cost of visual studio, but the time to recompile, test, distribute and fix any problems encountered.
>>
>>I've upgraded half-a-dozen or so projects from VS2010/.NET4.0 to 4.5 on VS2012 and can't remember any problems at all. Can't say whether going to VSE2012 would be equally straightforward tho.....
>
>One major problem is that I don't think MM.NET has been updated to 4.5 yet, but I may be wrong.

If MM is using .NET 4.0 it shouldn't be a problem - 4.0 & 4.5 both use same CLR (i.e. 4.0 & 4.5 assemblies can be used in the same solution)