A water taxi from Marco Polo Airport, for 1 to 4 persons, costs either �98 or �118 depending upon which city centre hotel you're travelling to:
http://www.venicewelcome.com/servizi/taxi/taxi -form-ing.htm
A land taxi would cost in the region of �40 to �50.
The (very frequent) bus service might be a better option:
http://www.veniceairport.it/page/servizi/trasp orti/autobus?m=01020202
As Dzug indicates, a land taxi will only take you as far as the Piazzale Roma (which is where the buses also terminate). From there you've either got to walk (or possibly take a water bus).
As has been indicated, Ryanair fly into Treviso airport, which is 30km from Venice. ATVO operate a fairly cheap bus service from there:
http://www.atvo.it/index.php?lang=en&area=23&m enuid=36
Chris