The problem is, we're talking about time as part of your budget. When you're an unknown artist, you cannot factor in your time. It's irrelevant because you're unknown and your income will be derived from people who have to decide if your product was "fun enough". In short, it's a business run on subjective personal opinion.
Hours to you personally, mean nothing in terms of budget or even profit, because almost every small business pays themselves last (and for good reason). If you are working with a team, however, their hours are important, and it's important to pay them what they are worth so that they stick around and continue to do the work they were hired to do. However, as basically the producer and publisher of the game, it is the customers who get to decide what your time is worth. If you spend 500 hours making a game and it sells 3 copies, your time isn't worth much of anything. But, if you spend 500 hours making a game and sell 5000 copies, that profit you just made is what your actual time, energy, and effort, is worth in real world terms.
You cannot really budget for "what your time is worth" as an Indie Developer because it isn't you who gets to decide that.
As for "Opportunity Cost", well, that doesn't really come into play until you're selling game after game and making pretty good profits on them so that you can actually quit your day job. Most Indie Developers do game design as a side project (like most any other artist). It's something they do in their free time, just trying to get noticed, just trying to sell enough to make a name for themselves, so that they can quit their day job. Eventually, you will make enough money to have to consider whether or not working a day job is worth more than being an Indie Dev. Most people don't get that far, however, so it's kind of a moot point. It isn't really up to the individual to decide what their time is worth (very few people on the planet get to ever decide what their OWN TIME is worth, because the market simply doesn't work that way in any country, because that's not how real life works), it is up to the person paying you to spend that time to decide what it is worth to them. If you don't think what they pay you is enough, you find a job where you think they will pay you more. If nobody will, then you have unreasonable expectations about your own worth. Your time is worth whatever people are willing to pay you for it. As such, you cannot really factor it into a budget. Pay yourself $50 an hour all you like, doesn't make it true, and doesn't make the finished product any better. Paying someone else that much, however... well, that's again subjective personal opinion. What is their time worth to you?
Furthermore, you probably shouldn't be using your "revenue the product brings in" as your base for the budget either. The budget should be what you're willing to spend to get to a finished product, and initially should be funded by your day job. Later budgets can take profits from the last game as their "budget", though I don't recommend doing that. I recommend using some of those profits in your new budget, and banking the rest in some sort of "business savings account" for emergencies and such.