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, 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.
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 and a stunning Italian villa map, now is as good a time as any to buy.