Let's face it, paint is expensive. While I think it is OK for beginners to use student grade paint, as soon as it becomes clear that they will continue to paint it's time to spring for the expensive stuff. Student grade has more filler and less pigment, possibly making it less economical in the long run.
One very basic way to save money on paint is to use a limited palette. The less colors you use the less you buy. That may sound nonsensical, but think fewer of the large economical tubes. If you mix your greens you won't wind up with numerous tubes of greens that you rarely or never use. That is wasted paint/money and I have an impressive pile. Many of mine were on the supply lists for workshops, sometimes not even used in the actual workshop and certainly not after. Others were colors I just HAD to have when I saw a beautiful painting and asked the artist what was used. That was before I had a better understanding of how color harmony and temperatures make paintings work, rather than "that color".
Another thing to try, both paint saving and a good exercise, is to mix your leftover paint from a session to make a "mother gray" for the next one, or even to use to tone some canvases. Using a gray you mix yourself can unite a painting and use paint you might have wasted.
Try to get the most out of every tube. For the studio I buy the large tubes and I wait for sales. As the tube empties I use a heavy duty tube wringer to completely empty it. This is the link to the one I use. Heavy duty tube wringer It is amazing how much paint is still in those large tubes when they appear to be almost empty. I use the metal wringer because I have heard complaints about the plastic ones. That said, the heavy duty is- go figure- heavy, so it stays in the studio. There is a light weight option for the small tubes I keep in my backpack for plein air outings. Check it out- Small tube wringer. It is handy but not as effective.
Once the tube is thoroughly wrung I don't stop there. I saw an instructor do this in a workshop and wondered why I hadn't thought of it-
Using pliers squeese the top of the tube all the way flat on both sides. See how much comes out of a small tube- imagine how much is still in a big tube!
You can also prolong the life of the paint on your palette. I always transfer leftover paint from my plein air easel to my studio palette when I get home to use later. This keeps my pochade box from becoming a dried up mess. I keep my studio palette in a plastic box with a lid. Though mine isn't airtight it still seems to slow the drying process. I have tried putting wax paper on top of the paint which helps but is a bit messy. I recently heard that Press and Seal is very effective and the paint doesn't stick as much. This is a grocery store product.
I never put my leftover palette paint in the freezer but apparently this works very well. There are various devices on the market to make this easy and convenient. Maybe I'll try it one day but my freezer space is limited. You can take a look at this one to get an idea- Paint Saver
For impasto work there are paint extenders that also add body to the paint to enhance the effect. My favorite is Gamblin Solvent Free Gel. In my experience it does not make the paint dry faster on the canvas but I love the way it adds body to the paint without diminishing its intensity. Of course you have to buy the extender, but it costs less than some of the pricier colors.
Professional artists likely don't need to worry about this. I know some who get free paint from companies they promote so they don't need to bother. (They have enough work to do as it is.) For me it is worth the extra effort.
This leads to one last thing- how NOT to save paint. Sorry to yell, but DON'T TRY TO SAVE PAINT BY BEING STINGY WITH YOUR PALETTE. You might as well put your paints away and not paint. Old joke, but so true... That said, pay attention to the colors you use in large quantities and those you don't. I have wasted so much alizarin crimson over the years by putting out too much. A little goes a long way. I put out a lot of ultramarine blue, yellow and white because I use large quantities of those. I use a lot less red and strong blues such as Prussian blue.
You can be economical with your paints, but paint liberally!