Because you are usually getting paid per project rather than on a regular salary, this can vary greatly. But unless you're a top lighting designer in Hollywood, it's probably going to vary greatly...on the low end. This is a tough business. Those working in theater can expect to make enough to afford clothes similar to the ones being worn by the orphans and paupers who populate the very musicals they are lighting. You'll need to really love what you do, because chances are you're going to need to do it a lot to survive.
The way in which you are paid also varies greatly from job to job. You might get paid hourly or you might get paid a flat rate for your work on the project, depending on the contract. If designing for a theatrical production, your take may simply be a percentage of the profits. No matter what, you’ll be making more than the lowly technicians and electricians working beneath you. Which isn't saying much for them.
If you want a number to go on, we'll give you one—around $45k on average, but don't go putting in a down-payment on that Winnebago just yet. Some lighting designers make over $100k, others are barely tilting the poverty scale at $20k-$25k. There may be some years when projects are flooding in and you're in the money; others when pickin's are scarce and hopefully you've put aside some savings. If you don't, trust us—you will not be able to appreciate the sad irony when the power company cuts off your electricity.