An accessible system would not increase costs. In fact, it could be more expensive because you'd have to spend a good amount of time testing both the regular voting system, and the accessible voting system. If your voting system requires a separate machine for simple disabilities like vision impairment, you've already given more of a potential of discounting an already marginalized portion of the population.
Thankfully, the guidelines to ensure things are accessible are fairly easy to find, and easy to implement as long as you keep accessibility as a requirement, and not a suggested feature. For example, the National Federation of the Blind has a list of features that cover the spectrum of visual impairment on their website for free [1]. Groups for people with other disabilities, like motor impairment, also have guidelines which aren't that difficult to follow. Smart phones with touch screens and the like are used by people with visual and motor disabilities too, so this isn't uncharted territory, it's just a matter of asking.
Thankfully, the guidelines to ensure things are accessible are fairly easy to find, and easy to implement as long as you keep accessibility as a requirement, and not a suggested feature. For example, the National Federation of the Blind has a list of features that cover the spectrum of visual impairment on their website for free [1]. Groups for people with other disabilities, like motor impairment, also have guidelines which aren't that difficult to follow. Smart phones with touch screens and the like are used by people with visual and motor disabilities too, so this isn't uncharted territory, it's just a matter of asking.
[1] https://nfb.org/purchasing-voting-machine