Topaz sells its apps as subscriptions. When we checked, Gigapixel on its own was $149 a year, and Topaz Studio, which bundles all their apps, was $69 a month or $399 a year. Topaz's documentation says trial versions of Gigapixel cannot save images and that refunds are available within 7 days.
If you need to enlarge a handful of photos for a print or two, that is a lot of software. PhotoRescue Pro costs $9.99 for a month with 200 credits, which you can cancel, and 3 credits a day are free without an account, so you can check the result on your own photo before paying anything.