I don't think there is a too much, long as you love the parts of the bass you're leaving stock.
I have a pair of SX basses, one is a MM with replaced bridge, electronics, pickup and string retainer, mods cost more than the bass, love it completely.
The other, a starter pack P Bass(my first bass), which I replaced the pickup, added a bridge pickup, and a Kahler trem bridge, each separate part cost more than the bass, couldn't be happier with the outcome.