I'm a fledgling developer and am just starting to get my app out to specific
clients.  
I have been asked to bid on a project because my app already does over 90% of
what the client wants.  My question is with regards to pricing of the
proposal.

It seems like I should include a license fee for my app as it currently is in
the pricing section of the proposal.  If I don't do this, then they might
claim that they paid for everything as customization, even though over 90%
already existed as a product currently for sale by my company.  Ownership of
my app then could come into play (although other clients are using the app
currently, so I could easily argue and provide evidence that it was already
an existing product)

But I'm unclear on the customization part.  Clearly I have to charge for
hours expended to do the customization.  But who owns the customization? Can
I include that customization as part of my baseline app, which I sell to
other customers (I'm talking customized functionality not anything really
specific to the client like labels that they like on forms, etc.).

Even if I can't use the customized part, because they claim ownership, that
part is useless to them without my app.  So, I'm just not sure.

I'd like some input from experienced developers.  Of course it is understood
that you are not providing any legal advice and I need to do my own due
diligence as well as retain competent legal advice.  I'm just looking for
anecdotal experience.

Thanks.