Hi Riicky,
I am currently setting up 2 tanks.
1 Marine 90 Litre Nano Cube (Reef) with corals, inverts and about 3 or 4 small fish. My total running costs are Tank with Cabinet £225, Rerverse Osmosis unit £90 for RO water (a must), Powerhead £40, Sand £20, Live Rock £90, Salt water test kits £20+, heater £20, inverts (shrimps, snails etc) £40+ and fish £50+. Total approx £600 so far and this is nothing fancy mind you and the sort of equipment listed above is not the cheapest but also not the most expensive.
1 Tropical FW tank 120 Litres with cabinet £200, External filter £40, Sand £8. Fish and plants approx £40-50. Total costs £300 approx so far
Long term costs for marine tank will be approx £20-£30 per month with reef salt, food, electricity but nothing else.
Long term costs for tropical will be approx £10 per month electricity, home made CO2
There you have it. These are all approx costs but marine tanks do cost more, you have less fish and the water parameters have to be checked every week without fail otherwise you may suffer a crash and everything will die in your tank and you have to start all over again.
Hope this helps