If you are on Magento 1 and are in initial discussions with multiple agencies about initial cost indications, it's likely you have had some wildly differing figures. We have invested heavily in infrastructure over the last 3 years to drive our service up and costs down. The fabric of our whole operation lies within our proprietary application MDOQ.
We have very well established production processes for key areas including code/extension audits, establishing data migration requirements, feature gap analysis.
We already have heavily tested Magento 2 extensions further reducing customisation costs. These include highly customisable Mega Menu, Sub-Category Listing Pages, Advanced Checkout Success Page and many other non-commercial extensions.