I'm a pixel artist. I know there's no 100% accurate way to charge client tasks but what pixel artists can do is break down large given tasks into smaller tasks, get the specifics on the smaller tasks nailed down to the bone, then provide a list of task prices to the client. That way both sides of the partnership can hopefully come to an agreement up front while minimizing risk. 

What I still have trouble with however is charging for client services that don't have "small tasks" or actionable items. For example let's say we're charging concept art. I have no idea how much it will cost up front and I can't charge lines or pixels because they don't provide the client any value or show any progress towards the final result until it reaches the point of a sketch. You can maybe charge that afterwards but even then, it's still confusing, there's no initial agreement, and it's too much of a time investment so overall it's a big risk for the artist especially if they're still somewhat new. So I was wondering if there are any artists who have experience with this kind of freelancing. I'd like to hear your thoughts. Thanks in advance.