These are well-built themes with good vendor support, which means the work worth paying for is fairly specific.
Say which theme and version. "Symmetry 6.x, moderately customised, need a custom fit-guide section on product pages and a mobile nav fix" gets an accurate quote immediately. Your version is in Online Store → Themes → the theme's action menu.
Check the documentation and support first. Clean Canvas maintain thorough user guides and their support is responsive. A meaningful share of what merchants assume needs a developer is documented behaviour or a setting. Paying for something the theme already does is the most avoidable cost on this page.
Brief mobile-first, because your traffic is. These themes are built mobile-first and the merchants on them usually skew heavily mobile. Ask any developer to test and demonstrate on mobile devices rather than a desktop browser resized — those are not the same thing, and the difference shows up in the results.
Ask how customisations will be structured. Kept in separate section and snippet files rather than edited into core theme files. That single decision determines whether your next update costs $600 or $3,000, and Clean Canvas themes update regularly.
Ask for a live store on the same theme. Symmetry and Enterprise differ underneath despite the shared house approach. Someone who has worked on yours will move faster and quote more accurately.