The primary differences with gravel bikes are the tires with 700x38 on most, the drop bar profile, and the possible addition of a dropper seat post. A brand new Specialzed Diverge E5 gravel bike retails for $1,300 which is not cheap if one is thinking Wal-Mart bike prices, but it is by no means expensive. The Salsa Journeyer Sora 700c is sold by bike shops and sells for less than $1,100.
Hardtail mountain bikes work well on gravel and the tire sizes they use are common to a great many tires from Maxxis and others that are less aggressive and suitable for use on gravel. The highly regarded Specialized Rockhopper bikes start at $600.
Take the time to do a little research before you start whinging on which is always counterproductive.