I would say that a 55 gal. is a good starting point providing you have the
space.but as navarre said it is too broad to put a solid figure to.
but starting out with a reef will raise the costs big time compared to
a fish only, which can be set-up fairly cheap, as long as you understand
what you are doing.with corals come high-end lighting,lots of live rock and
live sand,a quality protein skimmer(a must in a reef set-up, optional in fish-only)
just to start.