Just so everyone knows, I live in Canada, so the prices and such that I am going to talk about are in Canadian Dollars and shipping times are Canadian since they ship it out of Vancouver, British Columbia.
Every month. They also offer an OwlCrate Jr. box for MG and younger books if you don't get enough with the regular OwlCrate box.
Monthly: $29.99 USD
Every Three Months: $86.98 USD
Every Six Months: $167.94 USD
Plus shipping, which for me, cost $10.59 USD.
I just looked at my credit card statement and it cost me $51.96 Canadian. Which is a BIT expensive for me but I am making money now so it's all good.