Example:

Hypothetically: I notice that if I go on google flights and say I want to go from DFW->MGA its $1000+..

lets say I live in DFW.

So if I tell google flights to do YVR-DFW-MGA the price is only $730 , can i just board the DFW flight from dallas without ever being physically in YVR?

I've seen this trend for almost all flights being discounted so long as your choose DFW as your AA central hub using multi-city option. If you are an AA member, it might be the case for other airline hubs.