Ah budgets. Every artist loves budgets. I do them all the time and it's usually the worst part of my day, but it is necessary. I generally just write down all the tasks needed, how long they should take and have a fixed price per day or week on each task. It's pretty simple in the end, but only because I've put myself in the situation many times already. It's very important to budget. Clients will usually want something done fast, so you have to have a lot of backup plans on how to go about that. Sacrificies are usually made and you have to make sure there's a gap for that sort of thing so you end up having to compromise on both sides. There has to be a bottom end price that you won't dip below and you have to have proof of work that shows the client what you're capable of for a price. I just try to have all the bases covered, work calmly with the client in coming up with the budget together.