Most WordPress RSVP plugins can be found for free. For example, WPForms which is the best WordPress RSVP plugin, comes with a free plan that you can use to create an RSVP form.
Similarly, other RSVP plugins also come with free versions that you can use to manage your events.
However, if you upgrade to a pro plan, then the average RSVP plugin will cost you $10-$30 per month.