かなり高いと思います

www.venicewelcome.com/servizi/taxi/taxi-form.htm

料金をきちんと出している所はあまり見あたらないのですが、
近場で50程度、長距離で100ユーロ前後というところでしょうか。

以下のホテルでは10ユーロの上乗せがあるとか。
Attenzione: per i trasferimenti ai seguenti hotels è previsto
un supplemento di 10 euro:
Venezia Centro Storico: Cavalletto, Bonvecchiati, Splendid
Suisse, Kette, Saturnia, All’Angelo, Colombina, Ca’del
Campo, Montecarlo, Caneva, Città di Milano, Gazzettino, San
Zulian, Serenissima, Scandinavia, Panada, Ca’ Dei Conti e
Duodo Palace

Laguna di Venezia: San Clemente Palace, Sofitel Sacca Sessola

他のところでは、
Unfortunately, water-taxis in Venice do are very expensive.
The rate for a ride from the airport to San Marco (around 30
minutes) is about EUR 85,00, for four people and four luggage
(water-taxis have meters). There are some surcharges (night
surcharge between 10pm and 7am and surcharge for each piece of
luggage and person over the fourth).

The high prices are explained in part by the cost of the
motorboats themselves (the better quality mahogany ones cost
up to 110.000 Euros).

運河に面したホテルでは、ボートで乗り付ける客が本当のお金持ちですの
で、これも高い要因ではないかと思っているのですが。