I keep track of how much time I spend on each of my commissions by using a site called Toggl. (Not a sponsor. I like Toggl because of their simple and clean interface.) I write down the total time spent on each piece in a spreadsheet, along with how much I was paid.
I categorize entries by type, for example "spot illustration," "double page spread," "book cover," etc. That way, I have a general idea of how much time it takes me to complete a certain type of illustration.
I know. This seems so obvious, almost not worth it's own blog post. I never hear artists talking about tracking time, but it's so helpful!
Thanks to my spreadsheet, when a potential client offers me a job, I can estimate how many hours it would take to complete that job. That gives me a foundation from which to judge the budget they are proposing.
Keeping track of this information has revealed some interesting surprises. Sometimes the projects from large clients with the biggest budgets actually turned out to pay somewhat poorly on an hourly basis, because I spent so much time on them. On the other hand, sometimes low-budget illustrations for small clients paid the most on an hourly basis, because they asked for few revisions. It's taught me to really value clients who are easy to work with. (But that's a blog post for another day!)
Anyway, what I'm saying is: keeping track of your time is an easy way to start gathering useful information that can inform your business decisions. Just use Toggl or some other time-tracking app. You don't have to only keep track of commissioned work - personal work counts too. The point is to learn how long it takes you to complete a drawing.