By the time you buy the oats, add the margarine and syrup and then, shock horror, add in the cost of the energy to bake them (and, to be pedantic, your time in mixing, baking and washing up)99p for 500g is pretty cheap.
as in, 500g value oats are about 35p, block of margarine...umm...50p ish..value syrup another, oh, um, 25p, then there is the leccy/gas...20p? All figures plus or minus, but you get the point.
If you use butter - Sainsburys value butter is £1-19 per block.
If, on the other hand, you want to take Bedder's perfectly reasonable approach that you want to know whats in them and add flavours etc, then 99p value trays is not for you.