I'm not sure about reproofing drybags.
Reproofing with stuff like Nikwax, Grangers and whatever does not actually waterproof things. It merely adds a hydrophobic layer causing water to bead on the surface rather than soaking in. That can have the effect of improving the performance of jackets where drops can be shed off but with drybags, they're subject to being rubbed up against the inside of a rucksack etc which quickly defeats beading. I would have thought you want them to be properly waterproof - PU coated and taped fabric with no holes or anything.
If a drybag is letting in water then IMHO you need to either find and patch the hole or, if the fabric is no longer impermeable, bin it - unless you don't mind it not being reliable. A reproofed bag might hold off water a bit better but it can never make it waterproof. I wouldn't want my down sleeping bag for example in a reproofed bag that previously leaked.
I tend to get my bags from Sports Direct where the often have them going pretty cheap. Not the lightest but yhey don't seem terrible.