How much would it be to draft Terms and Conditions for our App?

Yes, terms and conditions on a business website can generally be enforceable against consumers. The enforceability of terms and conditions on a business website against consumers is not straightforward and depends on various factors, including the type of agreement used, the presentation of these terms, the consumer's acknowledgment of these terms, and the specific provisions within these terms. However, the enforceability primarily hinges on how these terms are presented and whether the consumers were given reasonable notice of these terms. There are three types of electronic form agreements typically used by websites to incorporate terms and conditions: "click-wrap", "browse-wrap", and "sign-in-wrap" agreements. "Click-wrap" agreements necessitate the user's express assent to the terms and conditions by clicking a button that reads "I Agree" or some other method of explicit agreement. In these cases, the consumer is presumed to be aware of the additional terms and conditions and consciously chooses to proceed with the transaction. "Browse-wrap" agreements include terms and conditions either posted on the website, a hyperlink, or are accessible on the screen, but do not require the user's express assent. The validity of these agreements depends on whether a website user has actual or constructive knowledge of a site's terms and conditions prior to using the site. "Sign-in-wrap" agreements involve notifying users of the existence of the website's terms and conditions and advising them that they are agreeing to the terms when registering an account or signing in. Courts usually enforce these agreements when the notice of the terms was "reasonably conspicuous." This can include situations where a hyperlink to the terms and conditions is present on nearly every webpage of the company's website, or when the webpage from which the user indicates their assent contains a conspicuous hyperlink to the agreement. However, there are exceptions to the enforceability of these terms and conditions and much depends on the type of business you are and the specific terms and conditions that you impose on consumers through your website.

While the general practice is to at least provide users the notice for any material change. However, Companies often include such clauses to allow them to modify any terms without notice. The legality of such language depends on many factors: (a) the interpretation of such language in the terms, (b) the nature of the changes, and (c) the applicable laws governing such terms. However, many courts and regulators expect companies to provide reasonable notice of significant changes, even if the terms state otherwise.

Users can contest anything - the question is would they win. I urge you to get legal advice from a qualified attorney before imposing or changing terms and conditions - I see you have a lot of questions about TOC, and you're not going to get proper legal advice/guidance/review just by posting questions. (And, no, please do not ask for me - this is a specialized field, and I'm not qualified or experienced to help you, much as I would like to).

Whether or not oral (verbal) terms and conditions are enforceable may depend in part on how well the written contract makes it 110% clear that no changes can be made unless they're in writing and signed by all parties in advance. Absent that kind of language in the written agreement, or absent any type of written contract, verbal agreements can be enforced - it's more difficult, of course, than if something is in writing.

Because every type of situation needs to be covered to protect you. That cannot be done in a paragraph or two.