Until April 6th, with a purchase of at least 40,000 miles Alaska Airlines will give you a 40% bonus on purchased miles, i.e. a cost of approximately 2.11 cents/mile. Alaska Airlines Mileage Plan program is not very well known since Alaska is not a member of any of the three major airline alliances. However, you have plenty of famous partner carriers serving destinations across the globe to redeem on.
Alaska Airlines has codeshare agreements with some members of both Oneworld and Skyteam: American Airlines, British Airways, LATAM, Cathay Pacific, Air France, Korean Air, Delta Airlines etc. The promotions like this happen several times a year and in the last couple of years I do not recall of any offers higher than 40% bonus on the purchased miles.
How many Alaska Mileage Plan miles do you need to get around? Here you can find the award chart including redemptions from all the regions and a few examples (excluding airline taxes) of round-trip fares assuming your are purchasing miles @ 2.11 cents per mile and redeeming for flights from the USA:
USA - ASIA in Cathay Pacific Business Class is 100,000 miles (~$2,110)
USA - ASIA in Cathay Pacific First Class is 140,000 miles (~$2,954)
USA - NORTH ASIA in Korean Business Class is 105,000 miles (~$2,216)
USA - SOUTH ASIA in Korean Business Class is 120,000 miles (~$2,532)
USA - AUSTRALIA/NEW ZEALAND in Cathay Pacific Business Class is 120,000 miles (~$2,532)
USA - AUSTRALIA/NEW ZEALAND in Cathay Pacific First Class is 160,000 miles (~$3,376)
USA - NEW ZEALAND in Korean Business Class is 125,000 miles (~$2,638)
USA - SOUTH AMERICA in LAN Business Class is 90,000 miles (~$1,900)
USA - TAHITI in Air France Business Class is 120,000 miles (~$2,532)
I do not suggest buying these miles unless you have a need to redeem them right away. That being said, if you are in the middle of planning a trip and wanted to fly First or Business class cabin, as you could see from the examples above this promotion could be a very good value.
You can register with the Mileage Plan program first (you need to be registered for at least 10 days to be able to participate in this promotion) and check out the availability for the flights you need on their or partner's websites. Korean Air and Air France availability should be checked on Alaska's website, LAN you can check on British Airways website, and for Cathay Pacific either British Airways or JAL websites (both require registration but are free to join) will do the trick. Even if you see availability on the other websites you should call Alaska to make sure their agents can also see and book the flights you found before purchasing any miles.