Is the WoW Token worth it? The actual gold-per-dollar math
A WoW Token is bought from Blizzard for real money and sold to another player for gold, or bought with gold and redeemed for 30 days of game time. Whether that is a good deal depends entirely on which direction you are going and what your time is worth.
Buying gold with money
Blizzard sells the token for $20 USD (€20 EUR, £15 GBP, $25 AUD, ¥75 CNY, ₩22,000 KRW, NT$500 TWD). What you get in return is whatever the region's token price happens to be that day.
At an all-time US average of 267,237 gold, $20 buys you roughly 13,400 gold per dollar. But the token has been as cheap as 107,392 gold and as expensive as 402,952 gold, which means the same $20 has bought anywhere from about 5,400 to 20,100 gold per dollar depending on when you did it. That is a 3.75× difference for an identical purchase.
So the honest answer to "how much gold is $20 worth" is: check the price today, and compare it to the price history before you commit.
Buying game time with gold
The reverse trade is easier to evaluate, because the value is fixed: one token is 30 days of game time, which otherwise costs $15 USD on a monthly subscription (less if you pay for six months at a time).
The question becomes: can you earn the token's gold price in a month more easily than you can earn $15? If the token is 260,000 gold and you reliably make 300,000 gold a month from crafting, farming, or flipping the auction house, your subscription is effectively free. If 260,000 gold represents weeks of grinding you would not otherwise enjoy, it is not.
Note that the exchange is deliberately lopsided: you pay $20 to give someone gold, and they pay gold to receive $15 of game time. Blizzard keeps the difference. There is no arbitrage here.
The trap to avoid
The most common mistake is buying gold to fund a purchase you have not priced yet. Auction house prices move too, and often faster than the token. Before you convert dollars into gold, check what you are actually buying: search your realm's auction house for the item, look at the price per item rather than the stack total, and work out how many tokens the purchase really costs. A 20,000-gold-per-unit reagent that you need 200 of is 4,000,000 gold — about fifteen tokens, or $300.
Current token prices for every region are on the WoW Token page, and you can price any item on your own realm from the home page.