In short: Evaluation pricing tracks scope, and quotes vary because firms are pricing different amounts of work, which makes scope the conversation worth having first.
Evaluation costs money, and the number depends on choices you control. These pages cover what drives the cost, how to right-size an evaluation, and what a proportionate budget looks like.
A fundable evaluation section names the questions, the design, a defensible budget, and the evaluator, and it's written while the proposal is still being built.