It's not necessarily/always cheaper to prevent an ongoing disability than to save a life. Some diseases are more likely to kill you (or leave you unscathed if you survive) than to leave you alive but disabled. E.g. rabies is almost always fatal once symptoms have presented— it's far cheaper to prevent a death from rabies (through vaccination or PEP) than to prevent a disability (only ~7 people are known to have survived).

I don't know if that's true for malaria or not. It can cause definitely cause ongoing disability, and it's far less fatal than rabies. It would be a massive undertaking to properly assess all the possible routes to disability and the cost of preventing them.

But as an example... here's a very crude back-of-the-envelope approach looking at cerebral malaria, a complication of regular/garden-variety malaria which can cause ongoing neurological disabilities:

• Let's imagine a group of 10,000 under-five children, living in sub-Saharan Africa. Malaria incidence for under-5s in sub Saharan Africa is around 26%. So without intervention, we can assume that around 2600 of the children will contract malaria. (0.26*10,000)
• Cerebral malaria affects about 2% of people with malaria. So let's say around 52 of our children will develop cerebral malaria. (0.02*2600)
• Without treatment, cerebral malaria is invariably fatal. With treatment, 15-20% die. Of those who survive, 11% are discharged with gross neurological deficits. For simplicity, let's assume that all of our children receive treatment, though this obviously wouldn't be true in real life. And to balance that out a bit, let's say that 20% of them die. That leaves 41 who survive (0.8*52), and 4 who have ongoing neurological disabilities (0.11*41).
• Now let's imagine we were able to distribute bednets to those children. Bednets reduce malaria cases by around 45%, so this time only 1430 will contract malaria. Of those, only 28 will contract cerebral malaria. (0.02*1430). 22 will survive (0.8*28), and 2 will have ongoing neurological disabilities (0.11*28).
• That means our bednet distribution prevented two disabilities! But how much did it cost? GiveWell estimate roughly $18 to $26 for one additional child under age five to sleep under a net. So it would cost $180-26000 to protect our 10,000 children. Divide that by two and you get $90-130,000 to prevent a child developing a neurological disability due to cerebral malaria. 

This is obviously far more than the cost to save a life. Even if we focused just on deaths due to cerebral malaria, our imaginary bednet distribution saved 5 lives, at a cost of $36,000-52,000 each. And in reality, it would have saved far more lives— you can die of plain 'ole malaria, without developing the cerebral kind.

I don't know whether this would hold true if you combined data for all malaria-related disabilities. It's possible that it would be cheaper to prevent a disability due to malaria than a death due to malaria. Regardless, it's likely more complicated than you'd think at first glance!

As an aside, I'm curious why it's important to you to prevent a disability rather than a death. (If I'm interpreting your question correctly). Do you see death as morally neutral, and only care about the suffering people experience while they're alive? If so, you might want to account for the pain etc. involved in dying of malaria. Preventing that suffering is probably morally valuable, even if you're indifferent to the actual life/death.