I know quite a bit about cars in general, and I have never heard of the
term "brake valve," unless the term "brake proportion valve," was what
was meant. I did a Google search in your behalf for "Subaru brake
valve." That term was not recognized by any of the part retailers.

Regardless, there does not exist any components in any car braking
system that could cost $1300 in a car with only 29K miles on the
odometer. Suggest you go to a good independent auto mechanic shop that
has been in business for a number of years, and ask  their opinion.
Avoid the chain operated stores like Midas, Firestone, Goodyear, and
the like. Those stores will really over charge you for things you don't
need done. The dealers (sometimes called stealers) are pretty good at
that too. A dealer quoting $1300 is coming across as a stealer.

Your intermittant warning light could be due to a loose wire or a bad
sensor. Those are cheap to fix.

Doesn't sound too familiar. That doesn't mean it isn't the ABS system.
Anything COULD fail. I THINK there have been some bad alternators that
can cause brake and ABS lights to illuminate, though that symptom as
well as bad ABS I seem to associate with models years well before '03.

If the car drives well, perhaps spending a little at another good
mechanic's for a 'second opinion' would be a good idea.

Carl