We use an 18x24" Contex IQ scanner to archive our client's work. With the use of Photoshop's Photomerge, we can seamlessly stitch together multiple scans of the same painting up to 24x36". We charge $25 per 18x24in scan for this service and it includes minor editing to clean up the edges of the scan and get the colors looking right.
We use a local photo archiving service for larger originals and color gambits. This service runs $100-150 per original.