The cheapest option depends on how many people in your group. From the airport, you can take a bus (ACTV or ATVO) or a taxi to Piazzale Roma, and from there hop on the Vaporetto (Waterbus) to the Sant’Elena stop. Look for lines 5.1 or 6. The cost for the land bus is 8 euro. If you are a group sharing expenses, perhaps a taxi would work, but expect about 30 euro from airport to Piazzale Roma. From here, the Water Bus will cost you 7.5 euro. Or you can buy a combo ticket for both land and water bus for 14 euro. This would most likely be your cheapest option.