Follow up question, how do you handle the open-ended part? list a cost of per hour on the contract and then on the road let them know how many hours it would take?
What i do:
- Tell my client why it is not reasonable/impossible my company does it fixed price
- Tell him also how many time I expect that it may cost (a best guess)
- Agree on that expectation
A typical conversation is something like:
"I must make a price while I do not have the specs yet. On my experience I guess it will take about .. days if the specs are reasonable. Having a price per hour at ... this would make ... So i can't do this part fixed price, but i can agree that i'll notify you as soon as i expect it takes more time, so we can find a solution together..."
Aside: this is not how it goes into the contract, there we formulate it as price per hour - without any restriction.
Problem for your client is that his boss wants to know how much it costs: he needs a number. So help him with that..