As a freelance web developer, you should have this tool your toolkit already.
We are quickly moving into a mobile age, and you should be able to test on mobile platforms; this is part of your job as a developer.
-- Edit --
I don't believe it is the clients responsibility to purchase any of your tools for you, unless it is an extremely rare tool which you could never use again, in which case this requirement should be negotiated in the initial contract.
I see this as akin to buying the tools for the contractor(s) building your house. Would you like to pay extra thousands of dollars to outfit them with basic tools of the trade, such as hammers and drills?