When two quotations for a similar-looking peptide differ sharply, the cause is almost never a hidden margin. It is usually that the offers describe different material or different obligations. Work through six variables before comparing the number at the bottom.
1 and 2: sequence and purity method
Chains are built one coupling step at a time, and stepwise yields multiply. If a repeated step runs at 80%, ten repetitions give 0.8^10, about 10.7% overall; at 95% about 59.9%. These are illustrative numbers, not process data, but they explain why a longer or heavily modified sequence costs disproportionately more. Then check the method: a figure by area normalisation and one by external-standard assay are not the same measurement.
3 and 4: specification window and quantity
A narrow purity window with individually controlled impurities is a different manufacturing commitment from a single minimum figure. Quantity interacts with it: screening quantities, repeat bulk volumes and pack sizes each change the release and documentation work per gram.
5 and 6: document scope and lead time
One quote may include chromatograms, an MS trace, elemental impurity results and microbiological testing; another may list them as chargeable extras. Lead time matters too: a fresh production slot, an in-stock lot and a custom synthesis with method development are three different delivery commitments.
A simple comparison table
Put the six variables in columns and one supplier in each row, then mark any cell that is undefined rather than cheap. Undefined cells are where a later change order comes from. Ask each supplier to confirm the specification and document list in writing before samples are dispatched, and compare only the rows that are fully specified.
Send us a competing quotation with the specification and method fields visible and we will answer point by point, including which documents are included and which we would list separately.