To be perfectly honest... this doesn't sound like a good idea... randomly not tracking users, visits or behaviours basically just makes your tracking inaccurate and untrustworthy....
As much as people complain about GA data being sampled, at least the sampling is based on overall traffic patterns from ALL events that occur, giving focus to the most important pages and events across the board...
I mean, in theory, if you really want to pursue this, you could try using a random number generator to set a value (0 or 1 - if you want to track 50% of traffic - if they have a value, don't generate a new value use the existing) into the user's Local Storage, then only track people who have "1" set...
But depending on your implementation, there may be other ways to reduce server calls without losing tracking...
Depending on your implementation, you might be tracking inefficiently...
- I've seen implementations that fire a Page View, then multiple action tracking calls (one for each marketing element on the page that was loaded)..... there's no need for this... those elements could all be combined into the page view call...
- Cut out unnecessary click tracking... you can use Activity Map to great effect here... I get it, direct "out of the box" isn't great, the region information is hard to digest... but there is documentation about how to customize your site into meaningful regions
I suggest you first investigate the underlying issues that require you to reduce server calls, and evaluate if there are better ways to implement your tracking...
It the above doesn't apply, and you really just have a small contract that isn't sufficient to cover your page view traffic, perhaps you have to have a conversation about why the contract isn't enough... if you care enough about data to purchase Adobe, you don't want to actively make the data you collect untrustworthy....