In case it helps to put numbers behind the question, a recent Callan survey showed that ~81% of plan sponsors engage an investment consultant. Now to be fair, we can't assume that all of those consultants engage in RFP work on a regular basis. Similar to what a few folks reported above, our firm will either have RFP services baked into the annual retainer, or hire out on a flat project fee basis.
You'll also want to consider the sophistication of your committee: are they familiar with concepts like revenue sharing, fee levelization, limited vs. open architecture, etc.? If so, it may be within their skillset to perform the RFP in-house. If not, we would seriously advise hiring a third party to assist with the project.
Remember that your job as a fiduciary is to ensure that your fees are reasonable for the level of services provided. While reading through RFPs will help you understand what services are available, and at what cost, it's hard to quantify or measure the quality of those services without personal experience. For example, for plans similar to your size, most bids will propose offer custom marketing solutions; however, this could mean anything from slapping a logo on standard templates, to full customization of the website & print materials to your company's look and feel.