thats just too big of a question... tank size can range from our nano tanks-to several hundred gallons!!!! nano tanks SEEM less expensive, but they aren't.
My favorite size tank so far has been in the 30 gallon range: the temp doesn't fluctuate too much, evaporation is limited, and water changes are easier cause they aren't too huge. My first tank was a ten gallon, just too small... I had a 55, and downgraded BACK DOWN to a 29 gallon... my perfect size, I guess.
most people use a skimmer and live rock as filtration. thats the standard, and probably the easiest for a newbie to do. DON'T get a cheap skimmer: I love my CPR bakpak, cost around 100 a few years ago: and they last forever. the live rock helps with the filtration also... thats something to research. theres a MILLION threads on here, and about 99,999 of them the tanks have live rock (aka LR)
clean up=snails, hermit crabs, things like that.
pocket money-yeah, RIIIGGGHHHTTT. my current seahorse tank, iun perspective:
35 gallon hex-new woulda been around 100-got mine used, in trade
sand-5 bucks at home depot
live rock-150-ish from pacificeastaquaculture
skimmer-new is at least 100+ if you fall to temptation and get a cheap one, then a good one, it'll be around 150+
various other things to seed the tank: 50 (just my, well, junk)
fish=200 (that includes shipping)
lights=180
corals (softies-I had to have em!!!) 50
powerheads-oh, I don't even know, but guesstimate 50 bucks for stuff here
mistakes=100 we all make them!!!
salt-so far, in three months buying it from the LFS, its around 90