Yes! While we offer a predetermined set of options for different amounts of Gold coins to purchase, it is possible to purchase any amount you'd like - it does not have to be one of these predetermined amounts.
If you send a different amount of funds, the amount of Gold coins (and complimentary Sweeps Cash) you receive will be prorated up or down according to current cryptocurrency exchange rates. You will receive the amount that's commensurate with your payment amount at the time your payment is processed.
You can always confirm the amount of Gold coins and Sweeps Cash you received in your Activity History.
See here to learn more about how to make purchases, and here to learn more about cryptocurrency.