If you buy a complete running system off someone, you should be able to get in for $800-$1000 for tank, stand, sump, lights, skimmer, pump and all the plumbing, live rock and sand. Check Craigs List regularly for a good deal. That is definitely the best way to get started when you don't know a lot. Especially because the person will sometimes throw in test kits, refractometers, water change setups, etc., which all together turn out to be expensive and often are forgotten when budgeting. Fish can run anywhere from $10 to $1000 each, but the common ones are $20-$50 (trigger, tang, lion, snowflake eel, foxface). Depending on the fish you could figure ten or so in a 125.