I've seen this blog post : Why you should support Linux and Mac? and it kind of changed my mind about the subject.
I'm developing an application (something like a SEO tool, to analyse a website and recommend stuff) in VB.NET. Theoretically I can support Linux and Mac users with Mono.
I support mono for my binary serialization tool, protobuf-net - and I'm surprised that most of the users are coming from the mono side (perhaps partly due to the lack of DataContractSerializer in mono?). So I'd say it definitely has a viable user-base, but it adds pain: during this project I've reported multiple compiler bugs in the mono C# compiler; some have been fixed, and some I've just had to re-write my code with workarounds that are happy in all the different compilers.
So it adds a burden, too. You also have to avoid the "not implemented" traps in mono, which is a pain when the (older) alternative way is marked "obsolete" in MS .NET.
I would expect that burden to increase as you move away from the "core" .NET offering - so anything involving any of the UI implementations (including Web) is going to be different between vendors.
To determine if it's worth it you have to analyze what you gain from supporting Linux and Mac. E.g., will it earn you more money? Attract the customers you want? If you could nail down some figures, it might be easier to compare and decide what to do.