Rainbow Six Siege (opens in new tab) is free-to-play this weekend, as you may have heard (opens in new tab), and if you like what you see you can also buy most editions of the game for 50% off.
The only edition that isn't half price is the Starter Edition—but you don't want that one, anyway. Evan wrote a guide to all the versions you can buy here (opens in new tab), and the Standard Edition is your best bet. With the discount, it's $19.99/£17.49. For that, you get the 20 'launch' operators, and unlocking more is relatively cheap.
All the other editions, which unlock more gear for a higher price, are also 50% off, if you don't fancy grinding.
You can take advantage on Steam (opens in new tab), the Humble Store (opens in new tab) or the Ubisoft Store (opens in new tab).
I bought the Standard Edition recently (at a higher price), and haven't regretted it for a moment: it's the most fun I've had in a shooter in the past year. With a big update on the horizon, which will add new operators (opens in new tab) and a stunning Italian villa map (opens in new tab), now is as good a time as any to buy.