GTA Online has had its repeat mission payouts reduced by 50%, according to Rockstar which admitted the change wasn't made very clear in the latest patch notes.
According to the developer, the first time a players beats a mission, they will receive the full amount but subsequent replays will see a payout amount reduced by half.
"This is to keep the game balanced as well as encourage the exploration of new missions and content in the game," said Rockstar on the support site. "We understand players do like to enjoy a mission multiple times, so rather than remove the possibility of doing so, we've allowed replays of these missions at a reduced payout.
"Many players can get very good at a mission and beat it much faster in consecutive tries, so we've adjusted these payouts to match that case. This also ties in to our ongoing discussion of replay availability for NPC missions, in which we are taking community feedback as well."
A couple users noted on the forum that the same could be said for repeating races, as players could easily become familiar with the race layouts yet the payout wasn't reduced on these.
I don't play it, so I have no idea.
You can look over the post in question through the link, and have a look at the patch notes again while you are it.