A sales person works with an engagement manager to come up with a price. That price may be T&M or fixed-price. The fixed-price is determined by the EM scoping the work needed. The price isn't based on hours, but on other scope.
So for Project A, it may be $10k. For Project B, it may be $7500.
1.Create a rate plan in the product catalog for each fixed fee that is possible. This is favorable if there are a small number of fixed fees.
2.Create a 'fixed fee' rate plan and then modify the price during the quote to reflect the fixed fee agreed.