I think you save more gold by using those extra percentages of land for either more slave pits or even gold mines. If you don't run an outrageous amount of pits, the casualties are not that bad and you make lots of money as long as you refill them every 6-8 hours or so.
if you have 20,000 slaves you lose 200 per hour. Every hour 200*3,2 = 640 gold is lost due to slaves dying.
In this example of 20,000 slaves, which is a rather large amount, every 1 % hospitals would save you 8 slaves per hour. (1 % hospital * 4) equals a reduction of 4 % of your slave-casualties. 4 % of 200 = 8.
If you have 5 % hospitals that is only 40 slaves saved per hour. The income you gain is 40*3,2 = 128 gold per hour.
That's basically less than the production of 1 gold mine or 1 slave pit. Imagine how many hospitals you need to make up 5 % of your land when you have 20k slaves... Urrrrk not worth it!!!
(I think I calculated everythign correctly!