Estimate the cost to scan a stack of documents.
Scanning in-house looks free because no invoice arrives, so the comparison has to price your time. Total time is the scanning itself at the rated pages per minute plus preparation — removing staples, flattening folds, separating receipts — which for mixed archive material often exceeds the scanning time. Valuing those hours at what your time is worth produces a figure comparable with a bureau's per-page rate, and for large batches of awkward paper the bureau usually wins.
Time
Minutes = pages / pages per minute + pages / 100 x preparation minutes per 100
Comparison
In-house cost = hours x your hourly value; bureau cost = pages x price per page
Rarely for mixed documents. Rated figures assume clean single-sided sheets; jams, rescans and reloading typically cut effective throughput by a third or more. Use a measured rate if you have one.
It is the main reason to scan in-house regardless of cost. If the documents contain personal or commercially sensitive data, a bureau needs a written processing agreement and appropriate security — factor that into the decision, not just the price.