Quotation form with automatic calculation fields.

We would like to have a quotation form on our website that will automatically calculate totals. The idea is that by selecting a particular dropdown list option only the relevant other options will appear e.g. If A is selected B & C will appear. If B is selected then total cost is 50. If B & C is selected then total price is 75. Form has to be sent by email. The other point to note is that some items will appear but be non-optional (the tick box will be greyed out) e.g. company incorporation costs although the amount will vary.

No monthly cost please. One time fixed price.