If you are patient, Black Friday deals on RODI units from some of the companies mentioned above are some of the best prices of the year.
Note 1: When it comes to this hobby, don't be in a rush. Save up $ for good quality equipment if you need to. Quality equipment will last you for years to come whereas cheap items will cost you more in the long run. I learned this the hard way. Save yourself the headache. Good lights, good filtration, good skimmers...this stuff costs more money upfront...but it is worth it in the long run.
Note 2: Don't build a tank you can't afford. I see a lot of people who buy big tanks because they find cheap deals on used aquariums and then don't have the money to maintain them. Saltwater is not a cheap hobby. Plan accordingly.