A site audit can yield a lot of very valuable information to allow you to optimize your website effectively. But is the cost always justified? Perhaps not always, but I’d say usually, yes. A quick snapshot of your site can usually tell you if a deeper dig is called for.
If your mechanic proposes an engine overhaul before even checking under the hood, or your doctor suggests surgery before running any tests, you’d rightfully question their judgment, wouldn’t you? The same is true for a site audit. Any reputable SEO is going to have already taken at least a quick look at your site before making a recommendation for a site audit. If they’re quoting you a price before you’ve even shared your domain name, then run, don’t walk.