First of all, I don't get out of bed for less than $100/hr. As a consequence, I don't do any consulting because I'm too old to be worth that kind of money. But there was a time, many years ago...
You have to realize that you are trading away the only thing of REAL value, time from your life, in exchange for something as boring as money. You only have so much life, charge accordingly.
I would charge by the hour for everything I did. You can negotiate an hourly rate (keep it high, you're trading away your life) and then bill for everything.
Another problem: Are you going to get 1099s (IRS form reporting this income) at the end of the year? If so, some percentage of what you earned is going to the .gov. Back when I was consulting, more than 50% of my income went to taxes (State and Federal). So, here I am, working every waking minute with the .gov taking half and they didn't even send someone to help! Even at $100/hr, I'm only really making $50/hr after taxes and that really isn't worth getting up.
Your client will probably want to file 1099s so that they can deduct from their income the amount they sent you. Otherwise, they have to pay you and pay the taxes on what they paid you. Better to send the 1099 and recover the taxes.
It's business, treat it that way!