Funding an evaluation can be a challenge for clients, when it isn’t covered by sufficient external funds. Although many foundations do set aside grant funds specifically for evaluation, this practice is far from universal.
For most organizations, it is much better to request evaluation proposals within a specific budget than to ask the evaluator to set both the budget and evaluation plan. Evaluations can vary widely in their cost depending on the amount of detail and scope required. If your organization has experience with evaluation, there’s nothing wrong with getting multiple quotes to get the best deal on an established evaluation plan. However, if you need the evaluator to develop the plan, it’s better to ask them to design one that fits a set budget, and then choose the best of the proposals submitted.
If neither the budget nor plan are defined, many of the proposals you receive may be either too far outside your budget or not thorough enough to cover what you want to evaluate. That ends up being a waste of time for bidders, and you miss out on having more desirable options to choose from.
If you want your evaluator to develop a plan for you, a good rule of thumb is to set aside about 5-10% of the program budget for evaluation, with higher budgets for more radical or difficult to evaluate programs, and lower budgets for those that are tried-and-true and have easy access to data.