For small projects, my company charges 200 hours for 'Analysis and Design' whose goal is to allow us to understand the problem better and to quote a more reliable price. Even at the end of this phase, we still prefer not to commit to fixed price, so we ask for (actual_hours * hourly_fee). When the client is interested only in fixed price, then it is estimate + 30% since we are taking the risk -- this assumes we are comfortable with our estimates. If we have not performed similar projects before, we will probably refuse to accept the new business when payment is based on fixed price.