What does a metal port mean to them? How many files? How many abstractions? How wrapped up in special case performance logic are they?
As there are not enough people who can do this, there should be a high rate. There is also a lot of minutia on top of the literal coding.
There is a ton of power you need to get this done. This needs to be granted to you as a matter of the contract. Continually reconvincing them to do the correct thing as you go will be tremendously draining.
You should have contractual technical assistance on their side with adapting their build system. (“In addition to a Decisionmaker, Client will provide a build assistance engineer to help with the asset pipeline and other project specfic tooling as needed, within 48 hours of notice of a problem. Build assistance engineer will supply written documentation on portion under discussion after the meeting for future documentation.”)
If you do not have a current and past stable of test devices, a portion (33-40%) should be billed as a fixed cost for acquisition of new devices as you acquire them. They do not get them at project end; they are overhead. Setup of these devices for each project is entirely billed to them at your full rate
You should also offer daily/bi-weekly communication on tried approaches at adapting their system to Metal. This may take up to an hour per day out of your 3-6 hour per day of billable hours depending on work. This should often use words from the book “Waltzing with Bears”. This is about risk management in their project, revealing known obstacles, and prompting timely intervention/decisionmaking from them when required.
You should be portraying low spoon days where you beg off constant application of low effectiveness strategies as “work was not performed on this project”, and generally, this can be portrayed as “I do 5 hours of the right work over 25 hours of the wrong work whenever possible” to push back against pushy/overeager clients.
They should be supplying a “Test Staff” person/people who are developing a end-to-end test to detect acceptable/unacceptable divergences. This person should be developing a shared, written test plan for a verification test, in addition to generating adhoc tests atop that. If this person doesn’t exist, you will need to charge them for doing this as well. They should be running part of automated portions of this within 3 weeks, and be able to do non-automated runs of this at least weekly.
You should have contractual ability to get artists to modify some assets to be appropriate to metal. You will be unable to specify the cost of this part at first.
Lastly, your near final deliverable is a document explaining the changes made to their code conceptually and refering source documents.