See also: Sale and Special History
Bundle Sales Edit
$19.99 bundle sales are useful if you are going to buy pickaxes anyway. Essentially, you get your pickaxes for regular price with free tonic and silver. They may not be as good (in terms of getting lots of pickaxes) as Buy More Save More pickaxe sales, but they happen a lot more often so they're easier to catch.
That $19.99 bundle appears to give enough resources to get a strong start AND begin to earn more pickaxes. As a player, I purchased one bundle and eventually built up to a total of 400 pick axes (from 135) without any other purchases just through the careful use of treasure chests and minigames. It isn't clear if the game makes things easier to get a high number of pick axes if you appear to be willing to spend money, but my opportunities to get large pick axe prizes (50 or even 200 at one time) did not happen until after I'd made that initial purchase.
$14.99 bundle sales are less common, but the price can be great! If you get the right one, pickaxes alone are 75% off, plus you get tonic and silver. Check whether it's worth $59.97 or $34.97. If it's the higher one, go for it!
Buy More Save More/Gold Rush Edit
If you are going to buy something, try to wait for a Buy More Save More or Gold Rush sale. They are worth the wait. However, they only come around about once per month or less for each resource. If you get desperate for the resource, waiting may not be an option. You do not pay less money. Rather, you pay the same money and get more of the resource.
% Off Specials Edit
If you want to buy items with gold, it's worth your while to wait for one of these sales. They usually aren't 60% off anymore, but 40% to 50% off sales are common. What you want to avoid is unrestrained spending just because stuff is on sale. That gold is still expensive!