You can certainly use such a number in a business plan/proposal when you're looking for investors, but you shouldn't use it all by itself. The real question is what percentage of beta users are willing to upgrade to a paid version after testing the software, especially if you're going after the freemium or trial marketing model.
You could find out how many users are willing to upgrade after trying out by sending your current beta users a no-commitment inquiry/survey. Ask them to provide feedback about the product with a few short questions; among those, one yes-or-no question about whether they would be willing to pay a certain fee to continue to use it. If the results are good, you can use that percentage in your argumentation with potential investors, and you could even factor that into your financial projections ("x% of all trial downloads will likely convert into paying users").
Just my 2c, hope it helps.