Foster affordable competition. For services contracting, consider using a simpler model, more akin to construction tenders or to the Request for Quote (or Price and Availability) used for off-the-shelf commercial products than to the approach usually required for system procurement. If you want to specify the services solution in detail (and can accept the performance risk inherent in doing that), then just ask for staffing plans and pricing, rather than for a full-blown proposal. The lower cost to industry of responding to a scaled-back RFP should increase participation and focus bidder attention squarely where you want it: on the cost.
Tasked as Technical Authority on an RFP? Never done it before? Done it before but not entirely happy with how it turned out? This is one of six posts on what contractors hope you do in that position, but are reluctant to tell you to your face.