1. Create a workspace in your home
If you can't afford to rent studio space, the simplest solution is to work from home. Many artists find it hard to be productive at home, which is why they rent studio space in the first place. If you do decide to work from home, you should create boundaries and establish a separate working space. If you have a spare room, for example, make that your workspace and do your work there. It doesn't have to be a spare room, of course; it could be anywhere you like, so long as you're comfortable working there. If you live with others, make sure they understand that this is your space to work and that you shouldn't be disturbed while at work. Do everything you can to make sure that while you're in your workspace working, you can get as much done as if you were working in a rented studio.
2. Create a workspace outside of your home
If you're lucky enough to have a front or back garden, you can take advantage of this extra space outside your home to create a workspace. The best way to do this is to have a shed, which has the advantages of being separate from your home and cutting down on your transportation costs. The only major downside to this is that if you don't have a shed, it can be quite expensive buying and installing one, though you could consider it a long-term investment. Another downside is space, because sheds aren't usually that big. Before investing in something like a shed to work in, make sure you'll be able to be productive in such a small space.
3. Work outdoors
A lot of painters, especially landscape painters, do their work outdoors. Many landscape painters paint en plein air, capturing the beauty of the natural world as they see it. If you can't afford to rent studio space, consider working outdoors - it isn't just landscape painters who do their work outdoors. Even if you're not doing paintings of the world around you, what you see, hear and experience can be a great source of inspiration. The main problem with painting outdoors is of course the weather. There's also the cost of transportation to consider, depending on how far away you're going. Overall it's a lot cheaper than renting a studio and if offers you the chance to get some fresh air and see more of the world, rather than being cooped up in a studio with four walls to stare at.