As @justanotheradmin notes, you need more information about the DFVCP and your role. Here are some things to consider:
Does the plan sponsor have in hand the Schedule SB Actuarial Information and all of the related attachments for each plan year? If not, who will coordinate gathering this information from the actuary?
Does the plan require an audit for any of the plan years? If yes, does the plan sponsor have in hand the completed audit report for each year? If not, what role if any, might you have in working with the auditors?
When filing retroactively under DFVCP for more than 2 years past due, the plan will have to use a current year form to report the earlier years AND will have to use the prior versions of any Schedules that needed to be attached to the original filings for each year. See https://www.askebsa.dol.gov/FormSelector/ to get a flavor for how this works.
In some ways, this is the tip of the iceberg, so you may get drawn into things like determining if appropriate fidelity bond coverage was in place, or tracking participant counts, or filing Form 8955-SSA, ...
A reasonable fee not based solely on hours spent, but also includes recognition of the knowledge and expertise you bring to the process.