Goalie Monkey doesn't have the G5 senior pads listed on their site yet, but they have the GT2 leg pads on for $799.99, the glove for $249.99, and the blocker for $229.99, for a total of ~$1280. GT2 custom Pro set costs $2229.99, so a senior set is about 60% as much as a custom Pro set, about $950 USD less.
The difference between senior and custom Pro GT2 will probably be similar to the difference between senior and custom Pro G5, though that's speculation on my part.