The easiest way to answer the questions you've asked is to ask the people that might buy it. Everything else is just conjecture.
Running some Google ads is a good way to do this and it's a lot cheaper than developing a product blindly.
Run an ad for keywords people might search for your product. When people click on the ad, your page should consist of a "sales" page with an offer to opt-in to a list. However, note that the success of your idea will not only be based on the merit of your idea, but your ability to write good Google ads, and good sales copy.
However, although this may seem like a lot of work, it will be less time, energy, effort and money than blindly building a product that in the end, doesn't sell. And...just think, what does this process give you? It not only validates the number of people interested in your idea, but it gives you a customer list as a side benefit!
Using Google ads to drive traffic, create a sales page. Google ads allow you to split test the page so you can offer two sales pages with different prices. When people click on the buy button, the page will not check them out. Instead, it will take them to a product not available page with an opt in to email them when the product is available. You could even offer a special discount code to them for opting in.
Your last question depends on a great number of factors, so you will have to calculate it based on how much it costs to develop the product, how much it costs to run your ads, your conversion rate for Google ads, the conversion rate for your sales page, and of course, how much you charge.