To get the best accuracy, a clear view of the sky is vital. That is why surveying GPS receivers have large aerials on a poles, to get them above the surveyor's head.
The tiny GPS receiver in a cellphone, in the user's pocket, is far less accurate, but usually good enough for vehicle navigation, or finding which building the person is in.
I don't think that the cost of the electronics makes a lot of difference. The GPS receivers will say how many satellites they are looking at, how good the signals are, and how accurate the position is likely to be. The more critical the application, the more accurate the data has to be before it is used. For your autonomous vehicle, if you position the receiver high up and unobstructed, and don't let the vehicle move unless the signal is good, you will have it escape the boundary less often.