If you're looking for a full-size ATX motherboard for Ryzen that costs less than $100, Newegg is offering MSI's B350 Gaming Pro Carbon for $90, after a $10 mail-in-rebate.
As the model name suggests, this is based on AMD's B350 chipset, which sits one peg below the X370. You can still overclock with a B350 motherboard, it just offers a few less connectivity options and less PCIe lanes. You can read more about the differences here.
MSI's board offers up a pair of PCIe 3.0 x16 slots, one of which is reinforced, and three PCIe 2.0 x1 slots. For storage devices, it has four SATA 6Gbps ports and an M.2 slot for installing a faster NVMe SSD.
It's a decent all-around motherboard, especially for the price. You can grab it here.
Newegg also has a nifty combo deal that consists of an AMD Ryzen 5 1600 processor with Wraith Spire cooler and a Gigabyte GA-AB350-GAming 3 motherboard for $260 (after $20 MIR). You can grab that bundle here.