There are three different offers to buy these three resources and the amounts provided with each offer vary based on the storage capacity of that resource and also the amount of resources you currently have.
The first offer is to fill storages by 10%. This provides you with resources equivalent to 10% of your current storage capacity. For example, if your storage capacity of Gold is 4 million, this will give you 400,000 Gold if purchased. Note that if your storage is more than 90% full, you will be unable to purchase this.
The second offer is to fill storages by half. This provides you with resources equivalent to half of your current storage capacity, and works similarly to the first offer. If your storages are more than half full, you will be unable to purchase this.
The third offer is to fill storages completely. This provides you with the remaining resources necessary to fill your storages, and based on the amount of resources you have in the storage, the amount provided will vary. For example, if you have a storage capacity of 6 million Elixir, and have 2 million Elixir, purchasing this will provide you with 4 million Elixir. It cannot be purchased when your storages are completely full.
The prices of certain amounts of Gold is identical to that of Elixir. In other words, it costs equal amounts of Gems to purchase equal amounts of Gold and Elixir. Dark Elixir has a separate pricing and costs more Gems.
You can use the calculators below to find the cost of purchasing these three resources: