A good tow dolly designed for regular, continuous use, with lighting and brakes, will cost $2000 to $2500 through lower cost online retailers, as much as $3700-4000 from a full price RV store. Add a couple hundred for connecting cables, vehicle lighting equipment such as magnetic lamps, and drop bar and bal for the hitch receiver. Another couple hundred for brake controller and instalation if you choose electric brakes. Thus the cost, compared tonequipping for Four-Down, is more of a toss up.
$500 tow dollies are sold for occasional short tows of light vehicles, often without consideration whether the tow is safe or legal. They are not designed for extended use, may even be sacrificial, resold cheap after one trip. Use of low cost marginal equipment is common in the lower levels of the used car business, but I don't think you want to be there with a carvyou own and care about.
If towing will be an occasional thing, rather than buying a junk dolly, you might cosider renting a good (though probably well used) dolly from the likes of U-Haul or Ryder.