Price work by value delivered.
Value-based pricing takes a share of the value created rather than a multiple of hours spent. Ten to fifteen per cent is a defensible share because it leaves the client the large majority of the benefit, which is what makes the fee easy to approve. Scaling the fee by your confidence in the value estimate keeps the quote credible: an uncertain benefit does not support a fee priced as if it were guaranteed.
Value-based fee
Fee to quote = client value created x value share/100 x confidence/100; margin = (fee - delivery cost) / fee x 100
Ten to twenty per cent is common. Beyond that the client's remaining benefit shrinks to the point where the decision becomes hard to justify internally.
Build the estimate with them from their own numbers. A figure they supplied is far harder to argue with than one you asserted.
Less well — measurement overhead outweighs the benefit. It fits engagements where the outcome is significant and quantifiable.