// Create cache for UIImage objects with total max size of 2 MB, objects exceeding size of 150 KB will be ignored
self.cache = [[AKLruDictionary alloc] initWithCountLimit:NSUIntegerMax
                                      perObjectCostLimit:2 * 1024 * 1024
                                               costLimit:150 * 1024];

...

- (UIImage*)imageWithMagicText:(NSString*)magicText
{
    NSParameterAssert(magicText != nil);

    UIImage* image = [self.cache objectForKey:magicText];
    if( image == nil ) {
        image = [MagicClass drawImageWithMagicText:magicText];
        [self.cache setObject:image forKey:magicText cost:[image ak_bytesCost]];
    }
    return image;
}