You are making a mistake that many noobs make - assuming that a smaller tank will be cheaper and easier to learn on. In fact, the smaller the tank the more difficult it is to maintain. I have been keeping marine tanks for over 40 years, and have only had moderate success with a 10 gallon tank I had in my kitchen - conditions just change too fast in a small tank, and I know what I am doing. It is just too difficult to learn on such a tank since the only thing you learn is how to retrieve dead fish from the tank. As for cost, price the equipment for a 20 gallon system, and for a 55 and you will find only a small difference, mostly in the cost of the tank itself. Things like heaters don't vary much, so a larger heater for the 55 will only be a couple of dollars more than for a 20. Your biggest long term cost will be, by far, the cost of fish. I recall the first time a lost a $50.00 fish - I couldn't sleep for a week. So the least expensive way to go is with a moderate sized tank since your animals will survive far better.