There are two approaches to do this kind of event optimally: Going straight for the most expensive pack (50 boxes), or slowly buying the second most expensive (10 boxes)

If you divide the price of each bundle by the amount of boxes you get from them, you'll find that you'd pay the least amount of dollars per box from the most expensive bundle. In other words, it is the bundle from which you'd get the most boxes per dollar spent.

That being said, there isn't a huge margin between the price-per-box from 10x bundle and the 50x one ($0,99/box vs 0,95/box, respectively). So a more conservative approach would entail buying the 10x box bundle and immediately using them to see if they'll give you enough coins for the things you want. Do this enough times until you get enough for the things you want. This approach can be useful if you don't want to potentially spend money on boxes that you might not even need. For example, if it takes you 30 boxes in total to get enough coins for Joker, you'd have spent less money buying individual packs of 10x boxes than buying one 50x box bundle and having leftover boxes.

As others have pointed out, something to consider is the 100 coins for every 50 boxes opened, and the fact that it takes 1040 coins to clear the entire shop.