The ad rate for online advertising can vary by platform and type of ad. Most platforms will allow to you set a daily budget you want to spend. Use these resources to learn more about advertising on each platform and to get a sense of the average ad rate.
Use these guides from advertising agencies and streaming services to help estimate the costs of advertising on a streaming service. Amazon provides their recommended budget for the different advertising packages they provide.
The ad rates for TV and radio can be more difficult to find. The rate usually depends on a variety of variables (e.g. time of day, length of ad, number of repetitions, size of audience, etc) and therefore is typically negotiated with the station or media company.
The ad rates for print magazines and newspapers are typically easy to find. Many publications will post their advertising rates on their websites. Simply search for "media kit" or "advertising rate" and the name of the publication.
If it's not listed on their website, you can contact the advertising department of that publication to get a quote on the type of ad you're considering.