Cosplaying gets expensive, going to conventions even more so. Actually making money as a cosplayer is incredibly difficult, but each of these things allow you to make a little something back on what you are spending.
Cash back sites. My favorite is Swagbucks, but I have also heard good things about ebates. Mypoints is another, and it offers cashback on a lot of sites that Swagbucks does not. Useful sites that offer cashback through these programs include ebay, etsy, Hot Topic, and Amazon, but only on certain items.
Sell some stuff. This can include extra supplies, old costumes you do not plan to wear again, or designs and patterns. You can start by posting in Facebook groups, starting an online store, or setting up a booth at a local convention.
Offer to Commission items. This means that you would take special orders for your services. Do not limit yourself to just doing complete costumes. If you are skilled at props or wig styling, you may consider offering specific services. Here are some useful tips to consider if you think this is a possible path. https://thegeekyseamstress.com/2016/12/19/10-essential-tips-for-first-time-commissioners/
Blog or Vlog. I do not think there are enough resources out there about cosplaying, and that is one of the reasons I have started writing more about it here. With blogs and vlogs you can make money with Patreon donations, affiliate sales, and other sources of income. If you offer other services like selling or commission, a blog is a useful way to promote your services.
Professional cosplayers do exist, but most people put more money into their costumes than they will ever get out of it. That does not mean you can’t make things a little easier on yourself. If you do any of these things, please post a comment below.