hi, i haven't advised any F500 client yet but have managed ad budgets of $25000 quarterly, so see if i fit into your application criteria.
next and more importantly, in such cases of low conversion for high end clients the problem is in story that they are using to convert visitors into paying customers. what i mean is the value proposition is being perceived differently by both your client and their customers. while not throwing money on ads is an obvious answer to scaling strategy, but you need to measure various touch points before we could measure why they aren't buying, and it will also include A/B tests of several things like value proposition, ad copies, targeted client persona, easy to use product, landing pages etc.
can i check the current touch points (ad structure, copies, targeting, analytic data). so that i can advise something sensible. will wait.
Best, Gaurav