For mini accounts, they are 10% of the regular lots interest - though don't expect to get paid interest if you are leveraging 400:1 or 300:1 etc. It seems anything less than 50:1, you will have to pay interest whichever pair you long or short.
My advice - use less leverage - safer and you don't get blown out of the water as quick, and earns some interest while you're at it - may not be much for a mini account, but hey 10 dimes make a nickel.
In the Oanda website, they have fxcalculators to calculate interest paid or earned down to each unit and time frame. If you are using FXCM or Refco, switch to the simple dealing rates and th interests will be reflected.