Android

android.cs

public int AvailableStorageMB {
	get {
		var statFs = new AndroidJavaObject ("android.os.StatFs", Application.temporaryCachePath);
		var availableBlocks = statFs.Call<long> ("getAvailableBlocksLong");
		var blockSize = statFs.Call<long> ("getBlockSizeLong");
		var freeBytes = availableBlocks * blockSize;
		return freeBytes / MEGA_BYTES;
	}
}

iOS

※ Android 側で指摘のように int だと大きな値になると計算途中の値がオーバーフローする可能性があるので、大きな値を扱える型にする必要があるかもです。(コメント欄参照)

storage.mm

/*
 * ストレージ空き容量の取得
 */
int storageAvailableMb_() {
    NSArray *paths = NSSearchPathForDirectoriesInDomains(NSLibraryDirectory, NSUserDomainMask, YES);
    NSDictionary *dict = [[NSFileManager defaultManager] attributesOfFileSystemForPath:[paths lastObject] error:nil];
    
    if (dict) {
        int Mb = 1024 * 1024;
        float free = [[dict objectForKey: NSFileSystemFreeSize] floatValue] / Mb;
        return (int)free;
    }
    
    return 0;
}

storage.cs

	[DllImport("__Internal")]
	private static extern int storageAvailableMb_();

	public static int storageAvailableMb {
		get {
			if (Application.platform == RuntimePlatform.IPhonePlayer) {
				return storageAvailableMb_();
			}
			throw new System.NotSupportedException ();
		}
	}