I think that learning how to make effective use of any

database package, XML

or otherwise, is going to cost you a lot more than 25k (in

any currency)

before you even start paying for software or developing your

application.

Well this is definitely discouraging... What do you feel is
the required
expense? (I am not opposed to paying what is necessary...)

It depends on your starting point, and on what you are trying to achieve.
But you need to learn how to write [efficient] queries, how to drive the
API, how to configure the server, how to write schemas and do the physical
database design (e.g. allocating space, building indexes), how to do
database loading, how to set up backup and recovery, how to register users
and allocate access permissions, how to build interfaces with other systems.
A full set of Oracle manuals occupies several metres of shelf-space and it
will cost you $25K just to pay someone to read them. There are good reasons
why database software is expensive; a well-performing database serving a
substantial user community and deliverying high availability is a
substantial investment. Rolling-your-own using operating system filestore
may be a lot cheaper, but it doesn't scale.