At this level, rarely would you think a first time HC would dicker over the amount of compensation. Moreover, Kroenke isn't one to underpay his coaches. He pays for quality. Pretty sure if Asshole Face were a FA HC, the Rams woulda jumped at paying him $9M per. So, I'd be shocked if the Rams were trying to lowball him. Now, us fans might think along those lines because we're so conditioned to that old way of thinking (/shudder...those Frontiere days still linger like a bad fart), but I don't believe compensation would hinder any hire for a moment.
MORE importantly, the negotiations (and that doesn't mean that are contentious... sometimes, they BOTH have to simply find out the answer before they both can sign legally binding contracts...) may entail things like...
1) Length of contract. The Rams may want a shorter contract. McVay may want a longer contract, so ironing that out may be a point, though I doubt it. I can't imagine they'd NOT settle on Four Years. Three is too short and Five is too long for a first time HC.
2) Locking down staff. It's one thing to suggest staff. Even if the Rams LOVE his suggestions, two HCs have already been hired. They have to ensure they're available and willing to join the staff and at what general compensation. It's not like the entire staff is a package deal, because we've seen key guys back our or had last minute issues, but you can bet as thorough as Demoff is being, he's gonna make sure his first choices for OC, DC and the top assistants are available to sign ASAP.
3) Amount of control. Honestly, I don't think McVay will want too much control over personnel, but he's going to want at least to KNOW and to have some say. Very few coaches are like Arians who kinda likes hardly any say at all.
Beyond that, there may be some other issues (I've never been a part of these kinds of negotiations, so I'm sure there's probably something I'm missing, but I can't imagine any deal breakers)