looking over the specs of some of the boards on newegg, i only see pcie 4.0 as the new feature. the gpu does not need it. so basically it's the 4.0 m.2 slot you're paying for.
these drives start at twice what you pay for 3.0 drives at 500 gb models and it gets worse the bigger the drive is. not sure a "budget" builder is going to spend the cash for such a drive.
so in other words, unless you just have to have a 4.0 m.2 slot, then a b550 board is not really the "must have" chipset. a b450 would do just fine and looks like cheaper for now as well.