The cost difference between building it in and adding it later is the whole point. Choosing accessible colours at the design stage costs nothing. Changing them afterward means revisiting every page, every graphic, and anything printed. Writing a label for a form field takes ten seconds during the build and requires locating and editing every form during a retrofit.
The items that account for most real world problems are short and unglamorous. Images without text alternatives. Text with insufficient contrast against its background. Forms whose fields have no programmatic label. Content that cannot be reached with a keyboard. Video without captions. Those five cover the majority of what gets flagged and none of them require specialist work.
Test it yourself before paying anybody, because the first pass is free. Unplug your mouse and try to complete your own contact form using only the keyboard. Run any page through a free contrast checker. Turn on your operating system's screen reader for five minutes. Each takes minutes and finds more than an audit would tell you at this stage.
Where cost genuinely appears is in a full audit and remediation of an existing site, which for a small business runs into meaningful money and is rarely the right first step. The proportionate approach is to fix the obvious failures yourself, build accessibly going forward, and reserve a professional audit for the point where you have a legal requirement or a genuine user need.
Understand that the legal position is not merely theoretical. Businesses considered places of public accommodation have faced claims over inaccessible websites, and the volume of such claims has risen. That risk is real and it is also frequently overstated by companies selling remediation, and the honest position is that basic compliance substantially reduces it.
Be careful with overlay products that promise instant compliance through a script. They do not deliver what is claimed, they are widely criticised by the people who actually use assistive technology, and several businesses using them have faced claims anyway. The money spent on one is better spent fixing the underlying markup.
Consider the commercial case alongside the compliance one, since accessible sites are better sites generally. Captions serve everybody watching without sound. Good contrast helps anybody reading outdoors. Clear labels reduce form abandonment. Those benefits apply to your whole audience rather than a subset.
Then build it into how you produce things rather than treating it as a project. Alternative text written when an image is uploaded, contrast checked when a colour is chosen, and captions added when a video is published. That is the version that costs nothing, and every other approach is paying to correct decisions that were free to make correctly.
Ask any developer or designer whether accessibility is included before hiring them, because the answer separates people who build it in from people who would charge to add it. Somebody who treats it as standard practice costs no more and produces a site that does not need remediating.
Prioritise by what customers actually encounter, since a checkout, a contact form, and your main service pages matter considerably more than an archive nobody visits. Fixing the paths people use is most of the practical benefit for a fraction of the effort.
Document what you fixed and when, since that record demonstrates a good faith effort and is worth having if the standard of your site is ever questioned.