In a small tank the cost of salt isn't that terrible in relation to weekly water changes. The larger the tank the more costly water changes become so the main benefit of a skimmer IMO is the removal of dissloved protiens in the water which helps keep down the levels of harmful gases such as Nitrates in conjunction with regular maintence.
The larger skimmer units also help to replenish oxygen - not sure about the smaller models like the HOB skimmer/filter combos or the air pump/ air stone driven lifts.
Although the start up cost of live rock can be intimidating, the good news is afterwards, take it slow, and you have plenty of time to buy what you need or want. Reefs are an ongoing project it just doesn't happen overnite. Even when buying a whole set up like the Nano, you'll need to immediiately get your head around - Salinity, Temperature, Feeding schedules, Media schedules, PH, P04, N03, Calcium, Mag, Alk, and slow acclimations to name a few. If you read the bottomline of the add the poster says " I'm unable to transport the Corals safely." Seller realizes it's not that simple, although I'm sure for experienced reefers it's do-able. Hence his last statement - "Serious and experienced reefkeepers only".
Best bet is pick your tank - add sand if you want, get your salt, mix it and test it in a 5 gal bucket. Line the sand with cardboard if doing sand, fill the tank with saltwater, turn on your heater, add pump, add a small maxi jet - one that doesn't turn your tank into a sand storm. Wait a day or 2 for the sand to calm down and the temp to be at least 77 degrees. Then add Liverock - let it run for a week and start testing for ammonia and nitrite. When ammonia is zero start testing for Nitrate aswell as Nitrite. When Nitrate is present turn on the lights for 8 hours daily and start testing PH. After 6-8 weeks when Nitrites are zero and Nitrates are 20ppm or less add your fish - then repeat testing - from this point your on your way and you'll have a healthy tank when the right steps are taken. Don't be discouraged when the color leaves the LR, a few months later it will return.