Here is my take on your question.
First of all "keeping cost low" really is not the best reason to use Free
softwares. Controlling one's own computing (having the four freedoms defining
Free software) is. It is worth educating the client about that.
You talk about "creating something proprietary to [your] company". If that
means the software is not distributed outside the company (and whatever the
number of computers in your company), no license is to be granted. Qualifying
of "Free" or "proprietary" does not make much sense... but you could somehow
argue that it is Free since the company (the only user) obviously has all
rights (hence, the four freedoms too) on the software it has created
in-house.
Pointing to proprietary softwares is not helping your client. Again,
educating the client is the key to make her understand why she only wants
Free software.
