This is just real quick from a budget I put together a while ago (tweaked for a 50 gallon tank):
Stand
50 Gallon tank
Overflow (unless included with the tank)
Lighting
Powerheads
Sump
Return Pump
Heaters (2 x 150w)
Temperature controller
Protein Skimmer
Test kits
Refractometer
Filters (socks, sponges, whatever you might want to filter particles)
Dry Rock
Live Rock
Substrate
R/O Machine
Water (R/O)
Salt
There are, of course, I would consider everything in this list non-optional. However, some things can be eased in to help with the budget. For example, using a traditional cycle (takes 6-8 weeks) you don't need the lights until the cycle is over; you can also skip the skimmer for the first few weeks as well (or the entire cycle). Both of which are a big part of the startup cost.
There are also lots of other things you are going to need (like nets, thermometers, light timers, etc), so budget in a couple hundred dollars for $5 items.
The sump is also, technically, optional. Which means you would't need the return pump at the very least.
I definitely recommend getting an RO/DI machine right away, it will pay for itself.