So normally I would also find cost per 1 roll etc. but if it is on a non-calculator paper then it will end up will tricky decimals/fractions which can be avoided. So we need to look for a way that deals with easier numbers.
Method:
First I tried 500/24 (to get no. packs needed per day) to see if that gave a nice number but it didn't.
Then I did (500 * 6)/24. 500buns * 6days is the no. buns needed per week and if there are 24 buns per pack (500 * 6)/24 gives no. packs needed per week. Luckily this comes out to give a nice whole number, 125.
Then we can just multiply no. packs needed by cost per pack, 125 * 1.99 = $248.75 ((This sum can be easier by doing 125 * 2 = 250, then 250 - (0.01*125) which is 250 - (125/100) = 248.75))
I've explained this in a bit of a convoluted way, but the general idea if you don't have a calculator is to try and find a way that deals with easier numbers. (If that doesn't work then it is often easier to work with fractions instead of reccurring decimals.)
Hope this helps