Your average cost for a double din made with any average reliability will run around $150+.
By the time you add in the necessary appurtenances you'll be similar cost. $30-$50 for a solid dash kit. $80 for a wire harness ($40 online). $15 antenna adapter. That will be your basic double DIN. Same cost and parts for a single DIN.
I can't speak to the cost and/or reliability of a cheaper brand/product. My HU cost estimates would be something like a basic Sony, Alpine, Kenwood.....etc.
There are for sure sub $100 units but I suppose it will depend on the features you're looking for.
Like I said, the above appurtenances are the same no matter which unit you purchase....it's up to you then to find the HU that suits your needs.