(I'm a newbie compared to most of you here, so ignore my 2c if you want) I'm going to agree with the others who have replied. I've recently rejected a job for a client who insisted that the huge changes they require are developed in the technology it is currently in - JSP. I basically gave them my recommendation of moving the existing tech into .NET and scaling it from there, or building on the existing tech with .NET, which they declined to do. My response was that I am not a JSP developer and that I simply could not help them with this project. ..long story short, I'm now developing an even larger application from scratch in .NET for them; it's completely unrelated to the above project. However I think they really appreciated that I didn't attempt to jump in the deep end, charge them ridiculous amounts for it and that I didn't push my suggestions on them. I now have a wealthy, stable client that is willing for me to come to the table for every project they require.
If you don't succeed, redefine success!