The cheapest way to get from Treviso airport to the center of Venice is by ATVO bus. The journey takes around 45 minutes and costs €6.55 per person.
Another option is to take a bus to Treviso Centrale railway station and then a direct train to Venice Santa Lucia, which will cost you 7,70 euros one way.
In both cases, you will arrive in Piazzale Roma or Santa Lucia train station, and you still must take the vaporetto #1 to San Marco Vallaresso.