You have some great answer. Even Micky may be right on the money in his own, trolling way.
I have seen on - page sold here for about $20 a page. That is a guy, with a team, adding images, bolding KWs, adding H tags, meta desc, and maybe adding related posts. If you were to outsource the project, it would cost at least $8000 if his service is not more now. Plus, I have to imagine this would take the team FOREVER to do.
A note not many people will think of is what KWs is your client trying to target. I hope I make sense here...
Customer is local selling 'Cars'.
Pages are about Trucks, Cars, Mini - Vans, ect.
Does the customer expect you to do KW research for LSI's for Trucks, Cars, Mini - Vans, ect? As in, do they want you to find and add LSI which they can rank for and get traffic?
This work load becomes a cluster f***, for the lack of better terms. That type of work is not only tedious, but very time consuming if it can even be done.
Make sure your client knows what they want and the KWs they want you to optimize for. If they have no idea and expect you to turn content into a ranking page, you may be in for a lot more work than a simple 'optimization' job.
If it is a blind, 'make it SEO friendly' job, I would not touch it unless I was being paid at least 5,000. This is assuming I need to make it work and do a LOT of KW research. Do not underestimate what a client may and will ask for. 400 page site sounds to me like it will not be as simple as adding "Red Tail Lite" to a product desc. I imagine they want more.