As far as I'm aware Distinct Corporation is the only provider of ONC RPC tools for .NET (VB and C#). See http://www.onc-rpc-xdr.com/products/rpc/rpc-dot-net.asp. I agree with you, the product is pretty expensive.
Your other option is to generate a C/C++ client stub as a DLL, and then use the Microsoft's .NET-to-COM wrapper technology -- Runtime-Callable Wrappers -- to call the DLL from your .NET client. See http://msdn.microsoft.com/library/default.asp?url=/library/en-us/dndotnet/html/callcomcomp.asp.
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