Of course it's getting cheaper. The legal stuff is cutting in on their sales, so they have to cut prices to keep customers. It won't happen until pot is legal in more states, but legal growers will drop their prices and run the cartels out of business. Walmart has been doing that type thing for years.
In the US, it's not an apples-to-apples comparison. Most legal weed has a higher percentage of THC than regular-grade street weed. If you want equal quality, you have to pay as much, or more, for illegal weed.
The super high grade stuff they are creating now (10-20% and more) is really just a waste of money. It may hit you harder for a while, but you soon develop a higher tolerance, and you end up smoking just as much, now at a higher cost. Smoking pot really needs followed by occasional periods of no smoking to allow the resistance to go back down. THC isn't really even the real game in town. It's a minor player. Truth be told, THC needs used in combination with another drug; together, the two fit together into a receptor site in the base of the brain to do far more for the "high experience" than either can do on their own.
That other drug? Sorry, that is information I don't give out.