To prevent backup server traffic from saturating the primary MPLS circuit, what inexpensive change should the engineer make?

Advertising a more specific route for backup traffic via the secondary circuit is the most cost-effective solution. Routers always prefer the most specific route available. By advertising a /32 route (host route) for the backup server's IP address through the secondary circuit, you ensure that traffic destined for that server will preferentially use the secondary path, offloading the primary MPLS circuit. Assigning traffic to a dedicated switch or placing servers in a dedicated VLAN only isolates the traffic at Layer 2; it doesn't control the Layer 3 routing path. Configuring a dedicated circuit is effective but significantly more expensive and not an "inexpensive change.