Thats not expensive. You might be able to get a higher class bolt at the hardware for a decent price.
(Metric doesnt come in grade catagory, so dont let anyone tell you that a grade 8 is the same as a class 8.8 - class is slightly weaker, but 8.8 will be alright...not great, but alright.)
The bolt size is 10x1.25mm (get stainless if they have them in that size and class)

McMasterCarr sells an 80mm long 10x1.25 class 8.8 in packs of 10 for $6.29, if you want to order and wait - but check the hardware stores first