Need to get sensor data from a Arduino Pro Mini to a Arduino Mega that is about 150 feet away from each other
Because cost was mentioned I'd consider the NRF24L01 based 2.4GHz modules. You can pick them up for pennies and some have reported over 100m range using external antennas so they may be adequate if you have relative line of sight.

They may or may not meet your requirements for link reliability, but like you said, it's all fun stuff and I've found these modules to be very fun to experiment with.