Assuming you don't have an audience but have a small budget. What I always do is buy a premium WordPress template already built for my target industry. I host it cheaply on godaddy.
I read all the competing websites and apps content and study what they think customers are looking for and what is important to them.
I then replace all the boilerplate text content in the template website to focus on my idea and how its better than others.
I create a simple contact form as the conversion goal and install Google analytics.
I then run a small Facebook and Google Adwords campaign with very low bids. I buy cheap traffic and send it to the concept website to measure a few things.
What is the cost for clicks? (CPC)
How many clicks does it take to get a conversion? (CR)
How much does it cost per converted lead? (CPA)
Once you know these metrics, you spreadsheet assumptions will be much more accurate.
You can even use WooCommerce or another Wordpress plugin to set up a single product checkout so you can actually measure sales. If you get any real sales it is just a couple clicks to refund the customer's money and apologize that the item is temporarily out of stock and you will follow up when it's ready.
Typically, I can completely validate an idea in any market for $1,000 or less.