I regularly take a multivitamin tablet that has (apparently) 200% of the RDA for Vitamin D (alongside a load of others), and have done for some time. Costs £8 for a tub of 180 tablets, so a little more than £16 for a year, or £1.35 a month. I don't spend much time outside, so this tablet does help. (I also take extra Vitamin C when needed.) It's rare for me to get ill, though this has been the case for a number of years. Somehow not missed a single shift at work due to illness. I don't know how many people take things like this, but surely it's cheaper in the long run to spend a small amount of money to have better general health.