Cheap means silnylon, something like

LiteOutdoors Silnylon Tarp - 11.6 Oz. - Ultralight Backpacking Tarp or Hammock Tarp https://www.amazon.com/dp/B07WTZDX1C/

And polycro under your bag, not a bivy,

Duck 286218 Extra Large Patio Door Shrink Film Window insulation kit, 1, Crystal Clear https://www.amazon.com/dp/B000NHW2Z6/