We have plenty of recommendations if you're looking for the best gaming monitor in any particular category, be it a G-Sync display or a 4K model. If your primary criteria is to keep things super cheap without going super small, good news, Walmart has a couple of interesting monitors on sale.
One is a 27-inch display for $79.99 (save $70 over list) and the other is a 32-inch model for $89.99 (save $80 over list). It's not clear what company actually builds these displays, though both are from Walmart's house electronics brand "Onn," hence the competitive pricing.
Starting with the 27-inch Onn monitor, it offers a 1920x1080 resolution, 60Hz refresh rate, and 5ms response time. There's no mention of amenities like FreeSync, G-Sync, or HDR, but we wouldn't expect any of those things at this price point anyway (not in the 27-inch category, anyway).
The 32-inch model is nearly same monitor, just in a larger physical package with a 6.5ms response time. Walmart also lists two refresh rates—75Hz in the title, and 60Hz in the specifications section. I'd err on the side of caution and assume the latter.
Bear in mind neither of these are likely to knock your socks off. They're not all that fast, and who knows how accurate the colors are. However, the pricing is rock bottom on these models. If you're looking for a monitor for the kids, or just want to keep things really cheap, these fit the bill.