ok grazie milleeeeeeeeeeeeeeeee!! sono riuscito..
posto se magari possa mai servire a qualcuno.

la funzione corretta:
function funzione(){
fh=document.forms[0];
a = fh.datoa.value;
b = document.getElementById("datob").options[document.getElementById("datob").selectedIndex].value * 150;
b1 = document.getElementById("datob1").options[document.getElementById("datob1").selectedIndex].value;
c = fh.datoc.value;
due = a * b * c / 5000;
tre = a * b1 * c / 5000;
if(parseInt(b)> 0){fh.output2.value=due
//calcolo 2
}else if(parseInt(b1)>0){fh.output2.value=tre
//calcolo 3
}if(fh.output2.value <= 20) {fh.output2.value = 18;
}else if(fh.output2.value <= 32) {fh.output2.value = 28;
}else if(fh.output2.value <= 42) {fh.output2.value = 40;
}else if(fh.output2.value <= 54) {fh.output2.value = 50;
}else if(fh.output2.value <= 70) {fh.output2.value = 65;
}else if(fh.output2.value <= 90) {fh.output2.value = 85;
}else if(fh.output2.value <= 105) {fh.output2.value = 100;
}else if(fh.output2.value <= 130) {fh.output2.value = 120;
}else if(fh.output2.value <= 160) {fh.output2.value = 150;
}else if(fh.output2.value <= 190) {fh.output2.value = 180;
}else if(fh.output2.value >= 190) {alert("Richiedici un preventivo email: servizioclienti@depurweb.it");
}
}

ciao a tutti ragazzi!