How can I be more frugal with salt when cooking (dried) pasta?

It's perfectly possible to cook pasta with no salt at all, so you may be able to wean yourself onto a lower salt level, reducing it gradually.

I don't know what, if anything, you're eating with your pasta, so getting that to have more flavour might be an option.

If you're actually eating plain pasta and want it salty but as cheap as possible, the first thing you should do is cook it in as little water as possible, and with a lid. This will also reduce your fuel bills (you can reduce them still further by turning off the heat for a few minutes in the middle of cooking). If you use half as much water as before, you could start by adding half as much salt, so the concentration in the water remains the same. To a first approximation, it's the concentration that matters here.

Even if you're eating absolutely at cheaply as possible already, I'd be very surprised if you could save significant amounts of money by using less salt, and if you can, you need to consider the health implications (I won't go into more detail). Think about how much in total you spend on it, because you can't possibly save more than that.

If you can't get cheaper salt, or reduce the ratio of salt/water, and you don't want to compromise the finished result, then the only option I see is to use less water.

Cooks generally gauge the correct amount of salt in pasta water by taste-- they say the pasta water should taste like the ocean. So, if you use less water to cook your pasta, you should likewise need less salt to appropriately flavor the water (and thus the pasta). This answer indicates that using less water should still yield the same quality pasta.

If your purpose is truly to save money, you can soak the dried pasta overnight, and it will cook much more quickly, as it will have rehydrated. This will lead to a similar cooking time to fresh pasta. Hence you will save on energy costs. To address the salt issue itself, just gradually add a little less each time you cook pasta, and you will soon find your taste adapting to a lower salt diet.

It can be a conundrum, either you need more salt or less water. You don't want to reduce the water as this can prevent proper cooking and cause it to stick.

Luckily salt will diffuse quite rapidly into cooked pasta, a fact I discovered when I once forgot to salt my pasta at all. You can drain almost all the water, leaving just enough to barely suspend and cover the pasta, before you add the salt. The salt will diffuse into the cooked pasta. Of course this doesn't happen instantly, and the pasta will continue to cook as long as it's in the hot water, so time it for when you are about a minute from completion (adjusting for the thickness of the pasta).

This will reduce the overall amount of salt needed, since you don't need to salt the full amount of water you cook with. It may also reduce the total salt intake, as the outer layers of the pasta will take up the salt more rapidly than the inner, giving some salt flavour but reducing the amount actually consumed. Mastication will eventually homogenize the salt content, but by this time the perception of saltiness has already been encountered.

Cook without any salt at all, and salt to taste at the table. Less will be used and none will be wasted.

Maybe a stupid answer, but what about buying more salt? Salt is really cheap in most places, so maybe you just feel like you're throwing money down the drain because you're using up your salt so quickly.

After removing the pasta, keep the pot of water on the heat until all of the water evaporates. Then harvest the salt. This will most likely not save you much money, though.

Alternatively, if you have enough time, and are in the correct climate, you could also set the pot out in the sun and let the water evaporate that way. I read somewhere it will take ~61 days to evaporate a 1-meter cubed amount of water in the Sahara Desert.

The salt increases the osmotic pressure of the water as well as flavoring the pasta. If you reduce the salt the pasta will not cook properly. Since the pasta water is typically used in finishing the sauce use the amount of salt called for in the recipe, typically 1 to 4 tablespoons for the gallon of water used for a pound of pasta. As noted in the comments above salt is literally dirt cheap. And do not add oil to the water. Just don't.

edit: ocean water has an osmotic pressure of about 27 atmospheres-- this effects the texture of the pasta

the pasta water that has a combination of starch and salt, when added to the sauce this helps the sauce to stick to the pasta (the salt controls the gelatinization of the starch)

and... you need a lot of water so that the temperature does not drop too much when you add the pasta