Since you have experience managing the account, you have a general idea of how much time you spend on it. Hiring an expert may or may not need more hours, than what you have been investing, but that is something you can suss out in a conversation and after they review the account.

Once you know how much time they'll spend then you just need to determine an amount that offers them fair value for their time based on their level of experience. You can either choose to pay them by the hour or you can do a percentage model.

As an agency, we charge a percentage of ad spend. In general, every thousand dollars of ad spend requires an hour's worth of work. This is non-exact as it also depends on how much testing and landing page optimization is being done, but it's a good rule of thumb.

The percentage, however gives us incentive to keep the spend profitable and ensure the client sees a solid ROI. The better the ROI the more they are likely to spend, which increases our income and gives us more hours to optimize the campaign for better performance.

The trick is to never increase your budget if you're not getting good ROI out of it.

Of course, campaigns that start on small budgets are trickier because they need time and data in order to be optimized. If your ad spend is low then consider paying hourly until the campaign is improved enough to increase your spend and move to a percentage model.

Here's an article on pay-for-performance models: http://marketingland.com/love-pay-performance-seo-wont-heres-209555. It focuses on SEO, but many of the same principles apply to PPC.